Down the Tubes with DTX!

DTX overview

San Francisco's Downtown Rail Extension project (DTX), officially known as the Transbay Transit Center Program Phase 2, is a two-mile tunneling project to extend the peninsula rail corridor from its existing terminus in the Mission Bay neighborhood to the purpose-built basement "train box" of the Salesforce Transit Center (SFC). The project is regionally important, as there are more jobs located within a half-mile radius of the SFC than within a half-mile radius of all Caltrain stops combined, from 4th and King all the way to Gilroy. The DTX is nearly shovel-ready, in the sense that environmental clearance is in hand and engineering is being advanced to award construction contracts the moment a key ingredient becomes available: money. Gobs and gobs of money.

Too Big To Fail

The last time the costs of the Phase 2 project were tallied in 2016, the total came to $3.9 billion in year-of-expenditure dollars assuming a 2025 opening. Due to delays, we can anticipate at least another five years of escalation at 5%, bringing us to $5 billion before any changes to the project scope. One can reasonably expect that Bay Area transit agencies' proven inability to deliver mega-projects on budget or on time is quite likely to blow up costs well beyond these figures. As a recent example, the Phase 1 project, completed in 2019, cost $2.4 billion (year-of-expenditure) or about 50% more than the $1.6 billion YOE budget of May 2010, adopted after the train box scope was added.

The DTX project's regional, state and national significance is certainly not lost on our Transportation Industrial Complex. To improve the chances of getting the Phase 2 project federally funded (after which any cost growth becomes easier to fund, following former SF mayor Willie Brown's "theory of holes"), the TJPA is undertaking a phasing study to make the project appear more thrifty. The various approaches include deferring or deleting components of the project, such as a pedestrian connector to BART, an intercity bus facility, and an extension of the basement train box. This nibbling around the edges amounts to $0.4 billion in 2027 dollars or about 8% of the total Phase 2 project cost, a drop in the bucket.

A $30 million project development study is now in the pipeline to get Phase 2 to the state of readiness required to apply for federal New Starts funding by August 2023.

PAX: The World's Most Expensive Grade Separation

If you thought the cost of grade separations is exploding, you really haven't seen anything yet: meet the Pennsylvania Avenue Extension (PAX) addendum to the DTX, a grade separation project that will approach $2 billion for two crossings, reaching the stratospheric cost of $1 billion per crossing.

Even after spending $5 billion (before inevitable cost overruns), the DTX project will leave two existing street crossings at grade, at Mission Bay Drive and 16th Street. Not to be outdone, the city and county of San Francisco has performed a methodical series of planning studies to conclude that a new grade separation project is needed. Rather than taking on the challenge of bending some design rules to keep it simple and make it fit, the favored paint-by-numbers engineering solution is a bored tunnel, which averts any conflict with a planned 27-foot sewer pipe and the sacrosanct pile foundations of the I-280 viaduct, each of which are under the jurisdiction of other agencies. The combined cost of DTX + PAX is estimated at $6.0 billion. Take away the latest (2016) $3.9 billion cost estimate of DTX and you get about $2 billion added for PAX.

Link21 Crashes the Party

Meanwhile, BART is in the early planning stages for beefing up its throughput capacity between the greater East Bay and San Francisco, with a second Transbay Tube. It's worth pausing for a moment to consider what an astonishing piece of infrastructure the first Transbay Tube already is: it carries almost twice as many people during rush hour as the entire ten-lane freeway that is the San Francisco - Oakland Bay Bridge, and at significantly faster speeds. Looking past the pandemic, long-term growth trends indicate that the region must plan for a second Transbay Tube.

Transbay Tube II is the centerpiece of an enormous regional rail program known as Link21, the scale and ambition of which dwarf the DTX. While there are many decisions yet to be made about the implementation details of Link21, perhaps the most critical decision centers on what technology to put in the tube: wide-gauge BART, standard-gauge regional rail, or both.

This question is already of great concern to TJPA, which writes in its August 20, 2021 Phasing Study:

BART and Capitol Corridor’s Link21 program is currently in the early stages of development and has not yet determined a preferred alignment, technology, or rail gauge options to meet their goals and objectives for a future transbay rail crossing. As expected at this stage of development, all options remain available for consideration. For example, Link21 may determine that a second transbay crossing best meets stakeholder needs if it provides additional capacity for the BART network only and does not provide a standard gauge rail crossing of the Bay. BART’s infrastructure and trainset design, however, are incompatible with Caltrain and CHSRA standards. Most significantly, BART operates on a wider track gauge with vehicles that may not meet collision requirements, and therefore a BART-only connection would not relieve congestion and conflicts on the DTX.

We can already see a problematic mindset emerging here, where "BART" is automatically conflated with "five-foot-six track gauge," setting up a false dichotomy of BART-or-standard-gauge.

Caltrain + BART: a Necessary Merger

The false dichotomy of BART-or-standard-gauge threatens to poison the debate around Link21 alternatives. The Transportation Industrial Complex has a vested interest in this incompatibility, as it ultimately forces multiple mega-projects to be built. Why build it right when you can build it twice and get paid twice? From the standpoint of scope and profit maximization, it would then make sense to keep DTX and Link21 as separate projects, despite their overlapping purpose and need to link the greater Bay Area megaregion together using high-capacity passenger rail infrastructure. Seamless integration is good for riders and taxpayers, but not so great for consultants and civil engineering mega-firms. That's why these firms have an interest in propagating the myth that BART and standard gauge rail will always be mutually exclusive.

Removing this false dichotomy is becoming a primary reason for merging Caltrain with BART to form a single Bay Area Rapid Transit system, although there are many other reasons. BART does not have to be synonymous with wide gauge; indeed, BART already operates a seamless standard-gauge extension between Pittsburg and Antioch, and provides day-to-day management of the standard-gauge Capitol Corridor. A new BART peninsula line, while indistinguishable from Caltrain's service vision, would suck the air out of the emerging pointless debate around the track gauge of the second BART transbay crossing. The Measure RR sales tax can serve as a dowry to integrate San Mateo and Santa Clara counties into a restructured BART district.

Link21, to its credit, places equity and inclusion at the forefront of its project development process. The contrast with DTX is jarring, as TJPA's Phase 2 project can easily be viewed as just another gold-plated white-collar rail project enabling nine-to-five technology and finance types to more conveniently access San Francisco's skyscrapers from the affluent suburbs to the south, without ever having to mix with the blue-collar working class. Bringing DTX under the Link21 umbrella, and merging Caltrain into BART, immediately defuses the classism and racism that underlies this anachronistic Mad Men commuter rail vibe.

Transbay Through Running

A stub-end terminal station suffers from fundamental throughput limits related to long turn times and the unavoidable crossing streams of inbound and outbound traffic in the station approach or "throat." For a given number of platform tracks, a through-running station configuration where all trains that come in one end of the station can exit the other end will always provide more throughput capacity, whether measured in trains per hour or passengers per hour. Trains don't have to dwell any longer than necessary at a platform, and don't foul opposing traffic on their way in or out.

With the DTX as it is, past operational analysis indicated that just 12 inbound and outbound trains per hour (8 Caltrain + 4 HSR) would push the limits of the terminus design, with near-saturated platform occupancy. If you uncork the other end of the train box (by having Caltrans clear some right of way i.e. dismantle and redevelop a couple of medium-rise buildings to the East) so that the DTX can connect directly to a new transbay crossing, everything changes. A lot of new capacity is created by virtue of not having to layover or turn trains right smack where your platforms and track real estate is the most expensive.

A recent through running operations analysis commissioned by the TJPA shows that the Salesforce Transit Center could handle up to 20 trains per hour per direction if no more than six of them turn at the station. Any more than six turning movements, and the excessively long platform re-occupancy times (as the study notes, due to the poor layout of the switches leading to tracks 1-4) will reduce throughput capacity to less than 20 trains/hour.

Broken Assumptions at Link21

The TJPA phasing study reports the following direct quote attributed to Link21 project team:

We have received briefings on the operational modeling for DTX and it would seem that even a three-bay DTX tunnel poses operational constraints. A robust service level through the transbay crossing is required to justify investment into Link21. Link21 is envisioning scenarios where not all trains that cross the Bay would continue to San Jose. At this point, there is no other location to turn trains around in the northern peninsula which makes flexibility in DTX important to the Link21 Program.

You read that right: the Link21 team is thinking of turning Capitol Corridor trains at the Salesforce Transit Center, a completely American idea (copied straight from Penn Station New York) that is operationally insane if you think about it for even a minute. In a through-running configuration, all trains that cross the Bay should stop in downtown San Francisco and get out immediately. The California High Speed Rail Authority is planning a huge yard in Brisbane, a perfect place to clean, service and layover Capitol Corridor trains. These deadhead (non-revenue) moves are much less wasteful of infrastructure capacity than treating a through-running station as a terminal.

As was remarked in previous discussions regarding San Jose, the act of parking or laying over trains at a station platform is the railroad equivalent of parking an empty truck in the middle of a bustling loading zone, and then concluding that the loading zone fails to function adequately. Just stop it, don't even think of turning trains here!

The Bottom Line

Here are the pros and cons of merging DTX with Link21:

Pro	Con
Eliminates silly idea of a multi-gauge transbay tube project	Could further delay DTX, since Link21 is at an earlier stage of development
Increases SFC throughput capacity and bang-for-buck, making the enormous cost of DTX worth it	Exposes DTX to political re-prioritization
Provides faster Peninsula - East Bay connections than existing BART, and finally "Rings the Bay with BART"	Greatly reduces scope and profits for Transportation Industrial Complex
Makes more efficient use of taxpayer dollars by building  one project and building it right	Requires inter-agency coordination and mergers, which agencies abhor
Provides seamless regional rail connection from SJ and SF to Sacramento, if Capitol Corridor is electrified

Despite the obvious political and organizational obstacles, from the point of view of a rider and taxpayer, the pros vastly outweigh the cons. The answer is then obvious: the DTX should go down the tubes of a new standard gauge Link21 crossing, with Stadler bi-level EMUs operated by BART seamlessly connecting the peninsula corridor (a.k.a. the new BART Purple Line) directly to Oakland and points beyond. DTX should be built without delay and form the first building block of Link21.

Billions of Seconds Wasted

The latest tweaks to the design of the San Francisco Downtown Extension (DTX) rail alignment can be seen in a March 2018 track plan and profile drawing. Because it largely follows the street grid, it's no secret that the alignment is full of sharp curves, which can only be traversed at slow speed. However, compared to a 2012 drawing, speed limits have dropped in several places from 40 mph to just 30 mph, because train speed evidently isn't a design priority when civil engineers get a blank check.

Back in 2012, the speed profile sort of made sense: starting from the basement of the Transbay Transit Center (left end of the diagram) the train would screech at about 20 mph through the sharp curve towards 2nd Street, speeding up to 35 mph along 2nd and through the curve towards Townsend. On that mostly straight bit along Townsend, speeds could pick up to 40 mph before dropping back briefly to 35 mph through the curve to 7th Street, then exiting along 7th Street at 40 mph (right end of diagram). If only one criticism were allowed, it wasn't clear why that final curve should be limited to 35 mph; there was plenty of space at Townsend and 7th to flatten it out to 40 mph, resulting in a simple and efficient stepped speed profile for the approach to Transbay.

Fast forward to 2018, and things are much worse. There is a new kink in the alignment where it connects to the existing tracks. The new underground 4th and Townsend station, at the city's request, has been shoved into the Townsend Street right of way in the hope of freeing up the existing rail terminal parcels for high rise redevelopment (where the 2012 alignment might have clashed with new building foundations). The rigid requirement for a straight island platform has resulted in a series of 30 mph kinks in the track. Elsewhere, the speed limit along Townsend has dropped by 5 mph.

The designers might argue this is only a few seconds lost, so no big deal, right?

How many seconds are wasted?

A train traversing the DTX will have to observe the speed limits not just for the length of each speed restriction, but for the added length of the train itself, as the limit applies from the moment the head end of the train enters a speed restriction until the tail end leaves the speed restriction. High-speed trains will be up to 400 m long, so this can really add up. We can simulate the time needed for a train to travel from a standing start at the end of a Transbay platform to a 40 mph entry into the existing Tunnel 1, a distance of about 2.2 miles. The results depend on the train type, and whether a stop is made at 4th and Townsend:

• 2012 alignment, single-length HSR: 4:04
• 2012 alignment, double-length HSR 4:17
• 2012 alignment, 8-car Caltrain EMU, no stop at Townsend 4:06
• 2012 alignment, 8-car Caltrain EMU, 30-second stop at Townsend: 5:04
   
• 2018 alignment, single-length HSR: 4:25 (+21 sec)
• 2018 alignment, double-length HSR 4:42 (+25 sec)
• 2018 alignment, 8-car Caltrain EMU, no stop at Townsend 4:27 (+21 sec)
• 2018 alignment, 8-car Caltrain EMU, 30-second stop at Townsend 5:19 (+15 sec)

To summarize and simplify, we can assume that every Caltrain will stop at Townsend, so the performance loss is 15 seconds per Caltrain movement, and roughly 20 seconds per HSR movement. That doesn't sound like much, but consider that trains are carrying hundreds of passengers, each of whom are individually delayed. The collective waste of time can be measured by multiplying the train delay by the expected ridership.

Today Caltrain has about 15,000 weekday boardings in SF, a number that Caltrain says could eventually quadruple. Let's say it only triples, and that 35,000 of those weekday boardings occur at Transbay and 10,000 at 4th and Townsend (which we won't count) making for 70,000 trips through the DTX approach. That's 70,000 trips x 15 seconds/trip = a million seconds wasted every weekday, or about 3 person-years of productive labor time per month of DTX operation. Over a year, about a quarter billion seconds would be wasted!

HSR eventually expects 18 million annual trips originating in the Bay Area, of which maybe half might involve Transbay. Combine that with a similar number of HSR trips terminating at SF, and you get 18 million annual HSR trips through the DTX approach. That would be a waste of another third of a billion seconds.

Every year then, about half a billion seconds would be wasted due to careless DTX alignment design.

How do we fix it?

Fixing it involves realizing that

1. every second matters, a lot
2. the marginal cost of the next second saved is more expensive than the last
3. saving seconds is most efficiently and cheaply done in the slow parts of a system

Making up 20 seconds through minor fixes to the DTX track alignment design, before any concrete is poured, is far cheaper and easier and more productive than trying to make up 20 seconds somewhere faster, for example in the Central Valley by running trains at 220 mph instead of 215 mph.

What ought to still be possible is an alignment that starts at 20 mph through the screecher to 2nd Street, rises to 35 mph along 2nd Street, then rises to 40 mph along Townsend continuing without slowing around the curve to 7th Street. With this improved speed profile, train run times from Transbay to Tunnel 1 (relative to the 2018 alignment plans) would be:

• Single-length HSR: 4:02 (23 seconds faster)
• Double-length HSR 4:14 (28 seconds faster)
• 2018 alignment, 8-car Caltrain EMU, no stop at Townsend 4:04 (23 seconds faster)
• 2018 alignment, 8-car Caltrain EMU, 30-second stop at Townsend 5:02 (17 seconds faster)

The combined annual time savings would exceed half a billion seconds per year. As we watch the cost of the DTX project reach ever more dizzying heights, we should at the very least expect to get more transportation value out of the project. Careless and inexcusable engineering of a rail alignment that wastes so much of everyone's time only adds insult to the injury.

New SF Caltrain Terminus Opens at 0 tph

Zero trains per hour (tph) is the inaugural Caltrain service level at San Francisco's new Transit Center, which opened to the public today after a decade of construction. The grand opening of the center, with its expansive $400 million basement featuring ghost tracks, ghost platforms and a ghost passenger concourse will no doubt crystallize the increasingly urgent transportation need for the downtown extension (DTX) of the peninsula rail corridor. Only then will train service increase beyond the current level of zero tph.

Huge opening day crowds at the Transbay Transit Center. Photo by Adrian Brandt.

Why build DTX?

Simple. Within a half mile radius of the Transit Center, there are more jobs than within a half mile radius of every station along the peninsula rail corridor from San Francisco 4th and King all the way to Gilroy, COMBINED! Even before high speed rail shows up, this is a piece of infrastructure that makes perfect sense. Or does it?

An epic opportunity for transit funding extortion

The clear (and, as of today's opening, agonizingly present) need for the DTX sets up a deliciously fat and juicy prey for the transportation-industrial complex, which you can think of as a hungry snake. Here we are, in a strong economy, in one of the richest cities on Earth, facing a specific and obvious transportation need: they can name just about any price. The latest estimate for the biggest meal that the snake can swallow is six billion dollars, and that's only the start. Scope creep, dizzying amounts of contingency cushioning, and construction change orders are sure to drive it far higher. Civil engineering megafirms, labor unions, and complacent and poorly coordinated government agencies are salivating at the prospect of feasting on the DTX. The bigger it gets, the more sated and comfortable everyone will be, with the notable exception of the suckers who pay taxes and ride trains.

The DTX project needs a major cost cutting exercise

"It is difficult to get a man to understand something, when his salary depends on his not understanding it." This insight by Upton Sinclair applies to any attempt to reduce the scope or optimize the cost effectiveness of the DTX project. There isn't and probably won't be a true will to do it, but in a pretend world where the interests of taxpayers and riders came first, where might you start cutting scope?

1. Delete the Pennsylvania Avenue tunnel extension. There is a perfectly serviceable tunnel already available. Engineering acumen should be brought to bear to overcome the (otherwise delightfully profitable) constraints of building a new trenched grade separation by figuring out how to shore up I-280 during excavation; how to cross the SFPUC's giant new sewer; how to duck under 16th street using a steeper 2.5% grade than the train people would prefer; and how to build temporary "shoo-fly" tracks under I-280 during construction now that the area is hemmed in by fresh UCSF construction. The usual paint-by-numbers engineering that deploys freight train design standards as "constraints" shows this to be categorically impossible, but is it really? Sharpen your pencils.
    
2. Delete the mezzanine level at 4th and Townsend. Station mezzanines are a knee-jerk (and delightfully profitable) design feature of every recent piece of rail infrastructure in the United States. Wedged above the tracks, underneath, in the sky or in a cavern, mezzanines tend to sprout everywhere. In this case, a mezzanine makes passenger access more circuitous and pushes the track level much deeper, increasing the depth of excavation. The mezzanine and station become an enclosed underground space, triggering an avalanche of fire safety requirements that greatly increase cost and complexity, with all manner of vent structures and evacuation shafts. The right answer is simple, direct and free-flowing access from platform to street, and an open station ceiling that vents to the street through a slot built into a raised median on Townsend Street-- as wide as necessary to treat the structure as an open station under fire safety regulations.
    
3. Daylight as much of the shallow Townsend Street portion of the alignment as possible, with a central median vent slot (just like in Los Angeles on the Alameda Corridor, where three of the nation's busiest diesel freight tracks are concealed beneath the street with a vent slot as narrow as six feet). This configuration has the potential to simplify the engineering considerations and costs related to fire safety, and even improves rail operations: without the onerous fire safety requirement of having only one train at a time occupy each tunnel ventilation section, operation of the entire DTX becomes less constrained.
    
4. Slim down the three-track tunnel, another one of Sinclair's salary considerations, to two tracks instead of the planned three. The Rail Alignments and Benefits (RAB) operations analysis, carried out by a premier Swiss rail operations consultancy, concludes on page C-68 that "Under normal conditions, only two tracks are required in the tunnel leading up to the TTC to operate the analyzed service plans. More detailed analysis is recommended to identify the most effective approach to provide infrastructure redundancy (e.g. the proposed third tunnel track) to help mitigate the potential effects of major service disruptions." The clear implication here, artfully worded so as not to upset Sinclair's salary men, is that a third track is not necessarily the best or only approach to achieve infrastructure redundancy.
    
5. Add three 400-meter underground storage tracks, feeding in towards the Transit Center instead of the peninsula, along the northwest edge of the existing 4th and King station footprint. The fire safety requirements for this underground infrastructure would be less stringent because it would not be occupied by passengers. With beefy foundation columns bored down to bedrock to straddle this yard, the entire footprint of the site can still be redeveloped above grade, safeguarding San Francisco's desire to use "value capture" from this increasingly coveted parcel to finance DTX construction. The resulting train storage capacity is far more conveniently located than the remote yard sites currently proposed at Oakdale or Bayshore, reducing long-term operating costs. Even skyscrapers can be built on top of train storage: see Hudson Yards.
    
6. Rationalize the Transit Center approach tracks to speed up train movements. The throat of the station has been identified as a key bottleneck for train movements (see RAB operations analysis page C-96, "Key Findings of Conceptual Planning"--and recall that you read it here first). An optimal layout has been identified that better enables concurrent arrivals and departures of two trains (see page C-117 of same). Precious seconds saved in the station approach can increase the traffic capacity of the DTX and make it more resilient to disruptions.
    
7. Don't use exotic and expensive tunneling methods when their sole purpose is to keep businesses along the DTX route healthy during construction, by avoiding cheap but disruptive cut-and-cover methods. The intent is noble, and the recent impact of Central Subway construction in Chinatown is painful and fresh in our minds, but this sort of thing rarely pencils out for anyone but Sinclair's salary men.

Only after a draconian cost cutting exercise might it begin to make sense to build the DTX. At a price point of six billion dollars for a couple of miles of tunnel, we regretfully should keep service levels at zero trains per hour.

Focus On: Mission Bay Grade Separations

The San Francisco downtown extension, as environmentally cleared and approved, ends at the existing grade crossing at 7th and Mission Bay Drive (formerly Common Street).  Somebody apparently forgot that this will leave grade crossings at Mission Bay Drive and busy 16th Street, where it still isn't exactly clear how to cross a trolley bus line with a 25 kV electric railroad at grade.  With train traffic expected to double in the next 15 years, conditions at those intersections are expected to become unacceptable, a problem which is now being considered along with redevelopment ideas for the Mission Bay neighborhood.

A number of radical solutions were proposed by outside groups (including the city!) coloring outside the lines of the environmentally cleared and approved DTX project, thus causing significant unease at TJPA headquarters.  To regain control of this issue, the TJPA commissioned a white paper study by engineering consultants Parsons Transportation Group and Jacobs Associates (organizations with considerable tunneling expertise), entitled Proposed Alternative Tunnel Connections to the San Francisco Downtown Rail Extension.

Grade Separation Tunnels Proposed

The billion dollar PowerPoint slide,
from the city's presentation,
used as pretext for dismissing a trench

Curiously, on page 1 of the white paper, the most logical option is summarily dismissed: 

• City SPUR Presentation Option 1 – This option consists of a trench trackway in between the I-280 freeway pilings. The presentation noted that this option may not be feasible, and therefore it is not discussed further in this white paper.
  

This seems like a rather weak rationale for not further discussing or studying a particular option: somebody else, presumably far less-qualified than the Parsons / Jacobs engineering team, happened to cast doubt on it.

One could cast a similarly dismissive eye on the entire study.  It boils down to considering various complex and over-wrought tunneling options (considering arcane subjects such as boring machine types, ventilation zones, and even tsunami run-up) in order to grade-separate these two streets for an estimated cost of well over a billion extra, compared to the current DTX plans.  That's right, BILLION.  Two grade crossings.  If you do the math, at least $500 million a pop works out to about ten times what a typical grade separation should cost.

The study's conclusion, not surprisingly, is to proceed with the TJPA's environmentally cleared and approved DTX plan so as not to upset or delay anything, with the understanding that nothing in the current plan precludes grade-separating in the future, if someone finds more than a billion under a rock.

Grade-separating a couple of streets doesn't need to be so ridiculously expensive or complicated, of course.  It requires an open trench built under the I-280 freeway, along the current alignment of the tracks and between the large pilings that support the freeway.  This trench would pass under Mission Bay Drive and 16th St, then climbing steeply (2.5% grade) to meet the existing portal of Caltrain Tunnel One.  No new tunnels would be built, and the freeway could later be torn down if the city so desires.

This modest 1/2 mile grade-separation trench extension would not require complex phasing; it would be built as part of the DTX tunnel, at an incremental cost far smaller than the proposed grade separation tunnels, with approval of a minor supplemental EIR.  There would be no need for fancy U-walls or tunnel stubs or temporary structures that would have to be modified at some future date with great disruption to train service.  The vertical profile would look very much like this:

Grade separation trench profile view (from west side of tracks)

There are a number of arguments raised against a trench at this location.

Constraint #1: Freeway Pilings

1987 grade separation trench concept

To see what a trench built between the I-280 freeway pilings might look like, we can set our way-back machine to the year 1987 -- more than a quarter century ago -- when Morrison-Knudsen Engineers performed a study entitled Peninsula Commute Service Interim Upgrade Study that included the I-280 diagram at right.  In 1987, the idea of building a rail trench under an elevated freeway wasn't simply dismissed without further consideration.

I-280 straddle bents at 16th Street

And suppose the trench did come too close to some I-280 piles?  The 1989 Loma Prieta earthquake triggered a massive seismic retrofit of the I-280 freeway, during which Caltrans demonstrated a surprising ability to add or move bents under an active freeway.  Even temporary solutions would be acceptable, since the city plans to tear down this section of I-280 in the short to medium term.  The biggest shame would be to spend a billion-plus dollars to avoid freeway pilings that are about to be dismantled.

Constraint #2: Major Sewers

There is already a large sewer passing under the tracks at Division Street (see profile diagram above).  Furthermore, the San Francisco Public Utilities Commission is in the planning stages of a $1 billion sewer project known as the Central Bayside System Improvements, which will involve tunneling a new 27-foot "Channel Tunnel Connection" underneath the tracks (see profile diagram).  The approved DTX project artfully avoids the sewers by surfacing before the sewer crossings, but a trench or tunnel would inevitably conflict with them.  The TJPA study assumes the new sewer tunnel forms the controlling constraint on the vertical alignment of a grade separation rail tunnel, which sends the rail tunnel so deep underground that it can no longer connect to the existing Caltrain Tunnel One.  That's what makes the proposed rail tunnels so long (all the way to Cesar Chavez!) and so expensive.

This raises an interesting question of underground right-of-way: should the rail tunnel yield to the sewer tunnel, or the other way around?  Without very much expertise in the art of pumping enormous flows of sewage, one might guess that sewage is more flexible than trains when it comes to changes in vertical profile.  The SFPUC and TJPA need to be prodded to come to a more realistic agreement on this issue, rather than flushing well over a billion of scarce rail dollars down that new sewer.

Constraint #3: Track Gradient

There isn't a lot of distance between the portal of Tunnel One and the 16th Street crossing.  If 16th Street is left where it is, the tracks need to dive about 28 feet down in that short distance, resulting in a rather steep grade of 2.5%, after accounting for vertical curves.  The TJPA study points out:

• insufficient length is available to transition from the tunnel below 16th Street to at-grade at Tunnel One while complying with vertical grade and curve requirements of DTX, Caltrain design criteria, or accepted railroad engineering practice.
  

Tunnel One North Portal

2.5% is not an impossibly steep grade for EMU electric rolling stock of the sort contemplated by Caltrain and the high-speed rail authority; portions of the high-speed rail system are being engineered with grades approaching 3.5%.  If there is a problem here, it is clearly with overly conservative DTX vertical grade requirements and Caltrain design criteria.  American "accepted railroad engineering practice" is totally irrelevant here, being firmly rooted in freight train operations.

If anybody tells you a 2.5% grade is impossible, ask for a second opinion, preferably from a European engineering consultant.  And there are ways to reduce the required dive: 25 kV overhead electrification clearances could be reduced by several feet by using rigid overhead conductor rails (instead of wires), or 16th Street could be raised by a few feet.  It sure is tight, but it most definitely can be done.

Constraint #4: Keep Caltrain Operating

Shoo-fly concept

Old wye track viewed from
under Mariposa, since removed

Pair of yard tracks to the east of the
main tracks, looking north from 16th

Keeping Caltrain operating during construction of the DTX connection is obviously a major concern.  It would be necessary to construct a temporary shoo-fly track on the east side of the existing tracks, to enable the trench to be constructed where the current tracks exist.  There are already some siding tracks to the east under I-280, but the challenge is to dodge out of the forest of freeway bents at Mariposa Street without violating side clearances between trains and concrete, along the path of an old wye track that was recently removed.  This can most likely be done as shown in the diagram at right; it is a puzzle that requires all the ingenuity our transportation industrial complex can muster.  If bents must be moved, then move them.  If clearances are truly insufficient, CBOSS can be used to enforce single-occupancy of the narrow section.

The TJPA study describes how the DTX project would be staged to allow tying in to a future grade separation tunnel.  The staging concept described would require disconnecting the Transbay Transit Center for several weeks or months while the new connection is made, with the 4th and King terminal used on an interim basis.  This staging concept is surely more time-consuming and disruptive than the trench tie-in work implied in the shoo-fly diagram above, which could probably be pulled off over a three-day holiday weekend.  Far more complex operations were pulled off on that time scale at the Bay Bridge.

Conclusion

Dismissing the trench grade separation alternative is premature and poorly justified.  Boundaries should be pushed and "requirements" should be questioned.  Coordination between the city, the TJPA, Caltrans and the SFPUC should be improved with the help of the Mayor's Office, and if necessary the Governor.  Most importantly, the small sliver of land between Mariposa and Sixteenth should be temporarily preserved from development until the DTX is completed, so that a shoo-fly can be built there.

Without a doubt, this trench would be a tough grade separation to engineer and construct, but when the only alternative is a billion-plus bored tunnel, there are suddenly a billion-plus reasons to take another look to make it work.  With the extensive means of excavation to be mobilized for the DTX project, the incremental cost may be small enough that Mission Bay and Sixteenth can be grade-separated concurrently with the DTX, without delaying the project.

The TJPA and its consultants should be sent back to the drawing board to study the feasibility and cost of the trench option in much greater detail.

Transbay Update

Early mockup of Transbay layout

Since our previous coverage of Transbay, a gigantic hole has been dug where the old Transbay Terminal used to be.  That's the part you can see.  Behind the scenes, design of the yet-to-be-funded DTX (Downtown Extension) from 4th & King to Transbay continues.  Millions have been spent to bring the design up to "30% engineering" where the details of tracks, stations, and tunnels have been sufficiently defined, down to the inch, to support construction bids and final design.

The TJPA was kind enough to share the latest overview engineering drawing (PDF file) with Friends of Caltrain.  This drawing is marked 'preliminary'.  Sadly, it does not reveal any substantive changes since 2009, and the DTX remains the same slow, 35 mph conflict-ridden mess that it was then.  Where previously it might have earned an overall grade of D, we'll give it a C-minus.  Some improvements were made around the margins, but certainly nowhere near what it could be.  Let's take a closer look.

1. Analysis of an ideal Transbay station interlocking

In the TJPA drawing, the Mission Bay station (built underground and alongside the existing 4th & King terminal) is laid out as in the diagram below.  Inbound traffic comes from MT2 (Main Track 2) and exits on MT5 or MT2, and outbound traffic comes from MT4 or MT2 and exits on MT4.

If we assume that MT4, MT2 and MT5 from a three-track tunnel that feeds the six-track Transbay Transit Center (TTC), we have 3 x 6 = 18 possible routes from the DTX tunnel to the TTC platform tracks numbered T21 through T26 from south to north.  Each of these 18 routes through the interlocking (also known as the "throat" of the station) is labeled by a single letter at the entrance and the exit. In this context, a route is one particular alignment of the tracks, established in the anticipation of a train using it to traverse the interlocking from one track to reach another.

Without getting into expensive simulations, a complex interlocking can be analyzed using a route locking table.  All table elements representing routes that lock each other (converge, diverge, or cross--i.e. conflict, and cannot be simultaneously set without risking a collision) are marked with an 'X'.

To evaluate the performance of the DTX track layout as proposed, we need to compare it to the ideal case, a track layout that cannot be improved upon because it has the fewest possible route conflicts.  If we assume that the entire facility must remain on one level (with no tracks passing over or under each other) then conflicts necessarily occur between any route that crosses over another, for example routes f and m in the diagram above, or routes that share a portion of track, such as m and n.  By inspection of the diagram, we can construct the ideal route locking table below.

Ideal route locking table for Transbay

This is the most conflict-free this interlocking design can ever be, and illustrates why stub terminals are avoided whenever possible.  234 out of 324 possible routes (72%) conflict with each other at best, so we would like a Transbay design that doesn't make an already bad situation even worse.

The route locking table can be improved (representing the pros and cons of various layouts more realistically) by weighting each route combination by the number of trains it carries, because we don't care if seldom-used routes happen to conflict.  Instead of filling each cell of route locking table with an 'X', we compute a weight that represents the relative frequency of route combination a,b as n_a * n_b / N^2, where n_a is the number of trains using route a, n_b using route b, and N the total number of trains.

The weights for the current Transbay design are computed by recalling that all Caltrain traffic is routed to/from platform tracks T25 and T26, while all HSR traffic is routed to/from platform tracks T21 through T24.  Furthermore, MT5 is intended for inbound traffic, and MT4 for outbound traffic.  MT2 is used more rarely to relieve congestion.  For the "blended" system planned for 2029, let's assume the following traffic levels in trains per hour:

• 6 inbound Caltrains, 4 of which use MT5 and 2 use MT2 to get out of the way of HSR
• 6 outbound Caltrains, all of which must use MT4 to serve Mission Bay
• 6 inbound HSR (of which 2 are dead-head moves with no passengers), using MT5
• 6 outbound HSR (again with 2 dead-head moves), 4 of which use MT4 and 2 use MT2 to overtake Caltrain at Mission Bay.

Out of those 6 x 4 = 24 movements per peak hour, ten use MT5, four use MT2, and ten use MT4.  Distributing that traffic to the corresponding platforms, we obtain the traffic-weighted route locking table for the "ideal" interlocking, subject to non-ideal platform segregation per TTC plans.  Because of this segregation between HSR and Caltrain platforms, each route is used exclusively by HSR (shaded in blue) or exclusively by Caltrain (shaded in yellow).

Ideal route locking table, weighted by traffic

The sum of each row or column is a measure of how conflict-prone each route is.  It comes as no surprise that the worst actors are routes q and r (because of high outbound Caltrain traffic) as well as e and f (because Caltrains inbound on MT5 must cut across the entire interlocking to reach their assigned platforms).  The 0.70 route locking rate means that two typical routes through the interlocking will conflict 70% of the time.

This implies the best Transbay track layout that can ever be designed will allow two trains to simultaneously move into or out of the station without regard to each other only 30% of the time.  Timetable planners and dispatchers will have their work cut out for them even on a good day.

2. Analysis of the Transbay station interlocking as designed

Inspection of the latest DTX track layout shows that it is configured like so:

The dotted lines show two curved 'emergency' crossovers, known as XO-201 and XO-202, that were provided in response to Caltrain's request for emergency operational capacity.  They will be used only when a failure occurs, and not during normal operations.  Without those crossovers, Caltrain was one failure away from total shutdown.

What immediately jumps out to a casual observer is that each pair of platform tracks (T21-22, T23-24, T25-26) necks down to a single turnout, preventing simultaneous routes from being established on both sides of the same platform.

By inspection of the DTX layout, one can draw up the traffic-weighted route locking table below.

As-designed route locking table, weighted by traffic

The cells highlighted in orange are those that differ from the ideal layout discussed previously.  The route locking rate has increased, to 0.75.  The unused emergency crossover XO-202 single-handedly accounts for 0.03, the lion's share of the increase.  The free-route rate of 0.25 is down from 0.30 in the ideal case; in other words, the probability of a conflict-free route is 17% lower than for the ideal layout.

For an already constrained and inherently conflict-prone stub terminal, it is unacceptable to make traffic jams even worse than they need to be, and yet that is precisely what the proposed DTX track layout will do.

The simplest improvement to be made is to add crossovers in the curved throat where the DTX enters the train box, to enable simultaneous and conflict free movements to/from opposite sides of the same platform.  This achieves the lowest-possible route locking rate.  The Caltrain curved crossover, XO-202 connecting tracks T25 to T24, is the most important one of these and needs to become part of normal operations.  Two more curved crossovers need to be added to allow simultaneous movements to reach HSR platforms.  The three missing crossovers are shown below.

DTX track layout with three crossovers added to achieve "ideal" route locking rate

Better yet, the Caltrain and HSR platforms should be desegregated to give dispatchers the flexibility to avoid conflicts when they do threaten to arise.  The route locking table gives the likelihood that any two randomly selected routes will conflict, but if a dispatcher can send a train to any platform or track, such conflicts can be avoided completely.

3. Another problem entirely: interlocking length

Route conflicts are one problem, but not the only problem with the DTX design.

When a train occupies the station "throat" interlocking, the duration of that occupancy must be minimized so that those conflicts that do inevitably arise (especially during off-timetable situations) are as short-lived as possible.  This conflict duration depends on the signaling system (through such parameters as route setting time, signal watching time, approach time, clearing time, and release time), and also on the time when the train physically occupies the interlocking.  This we can do something about; it is related to the length of the train, its speed, and the length of the interlocking.

As currently designed, the throat interlocking extends over a considerable distance from the TTC train box all the way down to Bryant Street. The November 2012 plans show the interlocking stretching from STA 148 to STA 173, a length of 2500 ft or 750 meters.  The speed is constrained at both ends, 35 mph at the south end and 22 mph into the train box.  We can take 29 mph or a nice round 13 m/s as a realistic average speed through the interlocking.

The physical occupancy time for one train is equal to (train length + interlocking length) divided by speed.  To this we can add 30 seconds of route clearing/setting time, and 30 seconds of approach time after the route has been set but before the train enters the interlocking.  For example, a 400 meter train moving at 13 m/s through a 750 meter interlocking will tie up a route for (400 + 750) / 13 + 30 + 30 = 148 seconds.

Using the assumed traffic levels as well as train length parameters below, we can run a simple randomized trial to quantify the relationship between throat length and the cumulative duration of route conflicts.  For each trial, the timing of train arrivals / departures is randomized over one hour (admittedly a worse assumption than reality, where conflicts are avoided by adhering to a timetable) and the total duration of conflict between routes is computed, after applying the route locking factor to account for routes that are mutually compatible.

Here are some realistic parameters for the "blended" scenario planned for 2029:

• Caltrain train length: 180 m
• HSR train length: 400 m
• Speed through interlocking: 13 m/s
• Caltrain movements: 12 per hour
• HSR movements: 12 per hour (including 4 dead-head movements to/from yard)

Effect of interlocking length
on average route conflict duration,
over the span of one hour

As can readily be observed in the graph at left, the relationship between the typical cumulative duration of conflicts and the length of the interlocking is strong, which implies that making the interlocking as short as possible will (a) facilitate the construction of a workable blended timetable by allowing closer spacing between train movements, and (b) facilitate timetable recovery after something goes wrong by reducing the cascading effect of delays in the station throat.

Note that things will get exponentially worse if traffic increases, as it may in the long-term horizon beyond the blended system.

As currently designed, the layout of the Transbay interlocking betrays little or no attempt to minimize overall length.  The resulting length of 750 meters is grossly excessive; it can and should be reduced to less than 400 meters.  A shorter layout for the interlocking would dramatically reduce route conflict duration--by fully one third compared to the current design--even if it requires the use of #10 turnouts and more turnouts and crossovers in the curved portion of the station throat, where the design speed limit is just 22 mph.  Why not take advantage of this tight speed restriction to do the dirty business of routing trains to the correct track?

This solution isn't obvious to American designers because it requires the use of non-standard track elements that may be considered exotic under typical U.S. freight rail AREMA standards, such as slip switches, curved turnouts and curved crossovers.  Indeed, this prejudice explains why the curved crossovers in the DTX design are usable only in emergencies. While designing "by the book" is always a safe option, it is unfortunately not feasible to engineer a compact throat layout for Transbay using standard turnouts.  Unlike the 100-ton coal hoppers with worn wheels anticipated by AREMA standards, this facility will only ever serve trains with very light axle loads, and maintained to the most exacting specifications.  Much of the conservatism and design margin built into AREMA standards is completely unwarranted for Transbay, and inevitably leads to a mediocre layout.

4. Conclusion

Transbay will be a special station, probably the most valuable piece of rail real estate west of the Mississippi.  As an inherently constrained stub terminal, it demands excellence in design to achieve the highest possible capacity. While the current track layout may be deemed adequate for the blended Caltrain / HSR scenario planned for 2029, the twin failings of unnecessary route conflicts and excessive length will rapidly reveal themselves as fatal weaknesses when traffic demand increases or minor incidents throw timetables into disarray.  Such entirely avoidable self-inflicted delays will cause excess traffic to be diverted to 4th & King, a terrible outcome for passengers as well as the economic vitality of the TTC and the city of San Francisco.  Minor changes to the DTX track layout can address these failings.

Progress at Transbay?

The many failures of the Transbay Transit Center design, as a train station, have previously been pointed out (see here and here). A November CAC briefing on the latest design iterations of the platform levels finally shows some incremental progress. After digging that $400 million hole, we may actually find signs of intelligent life down there.

• Caltrain tracks are moved to the northwestern edge of the train box, closest to downtown. While this change was advertised to avoid the foundations of the high-rise at 201 Mission St, (TJPA headquarters!) a better reason is that Caltrain ridership at Transbay will likely exceed anything HSR can muster. In this case, two wrongs have inadvertently made a right.
  
  
• With Caltrain tracks switched around, the future plans for tail tracks are now infeasible, and rightly so because underground tail tracks are the most expensive way you could possibly think of to park trains while they are not producing revenue. Again, two wrongs (the desire for tail tracks, and the competing desire for ramrod-straight platforms) make a right (no more tail tracks!)
  
  
• A new entrance is provided on Beale Street, with an escalator bank perpendicular to the train box, reaching towards downtown. Caltrain passengers can directly access the Caltrain platform without any scenic detours through the rabbit warren of sequential, airport-like functional spaces intended for HSR, which make absolutely no sense for the daily commuter. This new entrance is a big step forward because it suggests for the first time a concern with minimizing the overall travel time of Caltrain passengers. If it's not too much to ask, two more such perpendicular access points should be added at Fremont and 1st streets (better yet: emerging north of Mission, so pedestrians don't have to wait to cross those busy intersections.)
  
  
• Additional curved turnouts seem to have sprouted in the station throat, thankfully not labeled "emergency crossover", suggesting the possibility of improvements in operational efficiency and flexibility--although once again, nothing would beat a common platform height.
  

Happy Holidays to everyone.

Focus on: SF Transbay Transit Center

San Francisco's Transbay Transit Center, planned to be built in the years 2012-2018, will replace the decrepit Transbay Terminal and become the new transit hub of downtown San Francisco. The TTC includes a new underground rail station, which Caltrain and HSR will use as their northern terminus. The station will be connected to the tracks that currently terminate at 4th & King by a new downtown tunnel extension, also known as the DTX.

The DTX and revamped Transbay Terminal were planned since long before the high speed rail project, to better connect Caltrain and the peninsula to San Francisco and the east bay. Long starved of funds, the project was finally approved by San Francisco voters in 1999, with the creation of a new Transbay Joint Powers Authority (TJPA) charged with designing, building, operating and maintaining the DTX and the new Transbay Transit Center.

Fast-forward a decade, and Caltrain is playing second fiddle to the California High Speed Rail Authority, which (despite some past theatrics) fancies the Transbay Transit Center and its all-important underground train box as a northern terminus and veritable Grand Central of the West. Officials from the respective agencies are now squabbling over the design of the rail station, with the CHSRA's chairman Quentin Kopp calling the TJPA's design inadequate. (As it turns out, he may be right!) The disarray threatens the project's prospects for receiving economic stimulus funds, which has California politicians in Washington increasingly concerned that the appearance of "shovel readiness" is eluding their grasp.

The TJPA and CHSRA have developed a memorandum of understanding to govern future collaboration on the design of the DTX and Transbay Transit Center--not a moment too soon, since the TJPA plans to award construction contracts for Phase 1 of the project (possibly including the $390 million train box) starting in October 2009.

In this post, we'll discuss the rail component of the Transbay Transit Center and the DTX tunnel. We'll begin with a summary of the current train station design, as presented at the TJPA's March 12th board meeting (the source of most of the illustrations in this post), followed by some thoughts on what makes a good train station, and conclude with a discussion of shortcomings in the current TJPA design.

2013 Update: since this post was written in early 2009, the design of the TTC has evolved in small ways.  These are discussed in a 2010 update and 2012 update.

Many thanks to Richard Mlynarik for his advice and enlightening discussion of these issues.

Current Train Station Design

A cross section of the TJPA's design for the Transbay Transit Center (as of late 2008) is shown at right. It is a long, narrow building that occupies approximately the same footprint as the Transbay Terminal and its ramps (see location map).

The underground train station is reached via a tunnel which starts a bit before the existing Caltrain terminal at 4th and King Streets. After a ramp down and a sharp right turn onto Townsend Street, the tunnel takes a sharp left onto 2nd Street, then a sharp right into the terminal building.

Station Layout. The Transbay rail station has two levels: a mezzanine level (immediately below grade) and a second basement level with platforms and tracks, as shown in the plan at right by TTC architects Pelli Clarke Pelli. The mezzanine level does not extend for the entire length of the station; the south end contains a Greyhound bus terminal, and a large portion is reserved for 'program space'. What remains of the mezzanine level is further subdivided: a central area concentrates passenger access and contains ticketing facilities and some retail shopping, and is flanked by a Caltrain waiting area on one side and a larger HSR waiting area on the other side, which will likely feature airline-like passenger security screening.

Track Configuration. The station has six tracks, tentatively numbered 21 through 26, from south to north. The two southernmost tracks (furthest from downtown) are dedicated to Caltrain. The remaining four tracks are dedicated to HSR. Due to differing platform height and passenger security requirements, the tracks are permanently assigned to Caltrain or HSR and cannot be switched. Adjacent tracks are placed on 16 ft 6 in (5 m) centers. Two tail tracks were originally planned to extend beyond the end of the terminal; their layout has changed several times, but they are now "deferred until operationally required." Of the many tail-track options evaluated (including a loop tunnel back to 4th & King), one configuration required the southern track to pinch inwards to avoid building foundations, cutting the Caltrain platform short. The only vestige of the tail tracks is a bulge in the end of the train box, with knock-down panels where a future tail track tunnel might be built, if they ever turn out to be "operationally required."

Platforms. The station is planned with three island platforms flanked by a track on each side, 1300 ft (400 m) long and 30 ft (10 m) wide, curving sharply at the entrance to the station. The curvature (500 ft or 150 m radius) opens gaps in excess of one foot (0.3 m) between the platform edge and train doors. The platform height is 3 ft 6 in (1067 mm) for the level-boarding HSR tracks, and 2 ft 1 in (639 mm) for Caltrain-- the latter value seemingly picked to match the floor height of a Bombardier car. A row of columns supporting the terminal building above runs along the length each platform, spaced on 42 ft 6 in (13 m) centers. The assumed HSR schedule turn-back time is 30 minutes, with a nominal platform dwell of 20 minutes, giving the four HSR tracks a total throughput of 6 high-speed trains per hour (after accounting for the delay required for another train to re-occupy the platform). The Caltrain schedule turn-back time is 20 minutes (nominal dwell of 18 minutes), giving the two Caltrain tracks a throughput of about 4 trains per hour (or 6 tph with tail tracks). The TJPA claims it can accommodate Caltrain's planned TTC ridership of 31,700 passengers per day (versus ~8000 per day at 4th & King today).

DTX Tunnel. The downtown extension tunnel is designed with three tracks. The portions of the tunnel close to grade level are built using cut-and-cover methods, with the middle portion mined under Rincon Hill, until Folsom St. (curved section shown in TJPA drawing at right). The tunnel includes a new underground Caltrain station along Townsend St. between 4th and 5th, adjacent to the existing 4th & King Caltrain terminal. This station has two short 800-ft (240 m) outside platforms, with the center track used for overtaking trains stopped at the station. Since rolling stock choices for Caltrain and HSR are yet to be determined, the engineering drawings show clearances sized for Bombardier cars. Like the TTC, the new 4th & Townsend station features a mezzanine level above the tracks, requiring the entire alignment to be built at sufficient depth to allow for the requisite clearances.

4th & King Station. The DTX project preserves the existing 4th & King station location as a Caltrain terminal and train storage yard, since the TTC's two platform tracks will not support Caltrain's anticipated service levels or allow diesel trains (diesels may be used for Dumbarton service). Caltrain is planning for eight platform tracks at 4th & King. Their Caltrain 2025 plan, Appendix D, issued before Caltrain was assigned just two platforms at the TTC, assumes that fully 60% of Caltrain services will terminate at 4th & King. None of the platforms at 4th & King will accommodate HSR, due to the difference in platform height.

Curve Radius. The 1.4-mile DTX features three extremely sharp curves that allow the tracks to conform as best they can to the San Francisco street grid. The minimum curve radius is just 500 feet (150 m), the sharpest allowable radius for European off-the-shelf trains, and too sharp for some Japanese high speed trains. In theory, such curves can be navigated at a maximum safe speed of 35 mph (55 km/h); in practice, slower speeds of 25 to 30 mph are likely. Regardless of the exact speed, such tight curves will elicit deafening screeches from the stiff, long-wheelbase bogies that are a technical necessity for high speed rail. The DTX is likely to set California-wide technical requirements for high speed trains.

Station Throat. The track schematic above shows the configuration of the tunnel and underground station tracks, with the new underground Mission Bay station (under Townsend St.) also featured. The throat of the station, where the three tracks of the DTX tunnel divide into the six platform tracks of the TTC, begins all the way back at Bryant St., about 3800 ft (1150 m) before the bumpers at the end of the station platforms. At 25 mph, a train takes nearly two minutes to snake its way through the throat of the station. Most of the track switches are according to US freight (i.e. Caltrain) standards, AREMA #14's.

Passenger Access. The TJPA is sizing the station for simultaneous peak passenger flows of 5000 pax/hour for Caltrain and 4000 pax/hr for HSR. All vertical access (escalators and stairs) is oriented lengthwise along the station axis. Rather than accessing platforms directly from the street, passengers are funneled through a grand entrance and waiting areas in the mezzanine level. After accessing the platforms, passengers must walk some distance along the quarter-mile platform to their correct boarding location.

Grade Separations. Between the existing Tunnel #1 and the entrance to the new DTX tunnel, the Caltrain tracks cross 16th St. and Common St at grade. After the latter grade crossing, the tracks descend underground via a 1960 ft (600 m) open cut. A later grade separation of these streets would require demolishing this U-shaped open cut ramp and extending the DTX tunnel.

So What Makes a Good Train Station, Anyway?

Designing a train station for efficient operations is not rocket science, although space constraints can considerably complicate the exercise. The extremely constrained location of the TTC, combined with the very high level of train traffic envisioned by 2030, demand an intelligent station design.

We can draw up a simple list of three key requirements for a good terminal station design:

1. High train throughput. By definition, all trains arriving at a terminus must reverse direction. The station must be able to turn back trains at the highest possible rate; otherwise, the station itself becomes a worse bottleneck than the tracks leading to it.
   
2. High passenger throughput. The flows of passengers into and out of trains must be made as simple and direct as possible. This involves an optimization of street access, ticketing facilities, and vertical access to station platforms. Limited and circuitous passenger throughput can become a bottleneck at peak hours.
   
3. Operational flexibility. The station must provide flexibility to adjust service patterns in response to real-time conditions. It must degrade gracefully in the event of a service disruption, minimizing the possibility of cascading delays.

As we re-examine the TTC design, keep these three requirements in mind.

Shortcomings of the TJPA Rail Design

Throat Cancer. The TJPA station throat design is a bottleneck that promises low speeds, long traversals and frequent conflicts (when a key piece of track is occupied by one train, preventing other trains from passing.) The train throughput of a station throat, which sets a limit on the throughput of the station itself, is related to its overall length (shorter is better), train speeds (higher is better) and most importantly, topology (track switches thoughtfully laid out to allow conflict-free parallel moves.) The station throat begins a full 3/4 mile (1150 m) from the end bumpers, causing trains to occupy the throat for far longer than necessary and increasing the delay required for a platform to be re-occupied by the next arriving train, thus limiting train throughput. The station throat does not allow simultaneous arrivals and departures from most platforms (such conflict-free parallel moves are the holy grail of throat design because they minimize the re-occupancy delay), severely limiting train throughput and operational flexibility.

Different Platform Heights. With Caltrain's fleet due for a complete replacement with the advent of electrification, and with Caltrain's platforms due for complete reconstruction with the track expansion for HSR, the opportunity exists to make HSR and Caltrain share the same standard platform height. (The shared use of the same platform by HSR and commuter trains is common practice in Europe.) Designing the two systems for different platform heights makes this already-constrained station design even less operationally flexible because platform tracks cannot be assigned as needed to (a) minimize conflicts in the station throat, (b) accommodate service peaks, and (c) recover from disruptions. While this isn't TJPA's decision to make, the TJPA, CHSRA and Caltrain should agree on a single existing standard that is based neither on California's outdated safety clearance regulations, which were written for freight trains in 1948, nor on the floor height of Bombardier cars. Inventing new platform standards will hinder the ability of both operators to procure trains off-the-shelf and will create an operational mess.

Mis-Oriented Passenger Flows. The dominant pedestrian flows, especially for Caltrain, come from the center of San Francisco, which lies along the flank of the station (unlike traditional European terminals, which are often oriented toward the city center). Vertical access to the station platforms should be oriented perpendicular to the station, towards Market Street, to make use of the inherent horizontal reach of stairs and escalators to bring passengers closer to their destinations. Funneling all passengers through a grand entrance, concourse, fare gate, etc., all along the skinny axis of the building, while perhaps architecturally spectacular, does nothing for passenger throughput. Caltrain monthly pass holders need to get from the street to the platform, pronto, without any scenic detours.
Homeland Security Theater. The total segregation of passenger flows for HSR and Caltrain, to accommodate airline-style HSR security screening, constricts passenger throughput and prevents the flexible re-allocation of tracks. As the Madrid train bombings of 2004 demonstrated, securing HSR facilities does nothing to prevent mass carnage on the rails. Securing HSR without securing Caltrain is no more effective than building a moat only halfway around a castle. If security theater cannot be avoided, then adopt a security plan that does not operate the TTC as two separate mini-stations. With only two available tracks, Caltrain would find itself just one minor breakdown away from a total disintegration of their rush hour schedule, a disaster for operational flexibility.

Extremely Tight Curves. While curve radii at the location of the TTC itself are dictated by the street grid and surrounding building foundations, the two curves in the DTX tunnel approach (to turn from 7th onto Townsend and from Townsend onto 2nd) are much sharper than they need to be, and beyond the capability of some off-the-shelf high speed trains. High speed trains aren't MUNI street cars and won't turn on a dime; they are longitudinally stiff to provide good stability at high speeds. At these two curves on the 1.5-mile approach to the station, the radius could be easily increased from 150 m to at least 200 m, saving 30 to 40 valuable seconds in the long, slow crawl into the TTC, and increasing the train throughput of the station throat. If the throat can be straightened as well, a 200 m minimum radius would allow off-the-shelf procurement of trains without the need for expensive modifications to fit the extreme curves of the DTX, which threaten to set a standard for the entire California high speed rail system.

Odd Tail Track Arrangement. Among the various design options considered before trail tracks were de-scoped, no clear function can be ascribed to the tail tracks under Main Street. They could theoretically have been used to increase HSR train throughput by (a) performing cleaning and resupply of trains without tying up a platform and (b) switching departing trains to the northern platform, to minimize conflicting moves in the station throat during departure. In practice, none of the TJPA's attempted tail track layouts exploited these possibilities; they amounted to very expensive train parking.
Inefficient 4th & Townsend Layout. The new underground station at 4th & Townsend should have four tracks to allow overtaking stopped trains in both directions (to improve train throughput) and its platforms should be lengthened to 400 m to allow HSR trains to use them in the event of a disruption at the TTC, thus improving operational flexibility to HSR and Caltrain's benefit.

Oversized DTX Tunnel. Despite the high level of train traffic, a simple two-track tunnel into the station would do just fine. Because trains are limited to the same homogeneous speed by the sharp curves, a two-track tunnel could easily support 15 to 20 trains per hour, each way, or far more than the terminal could feasibly handle. The third track makes the tunnel needlessly expensive, and requires fancy construction techniques like the New Austrian Tunneling Method (NATM).

Suggestions for Improvement

All of the design issues enumerated above have been analyzed by San Francisco rail advocates, who have suggested numerous possible improvements for the DTX and TTC rail station in order to exploit the constrained site as efficiently as possible for both Caltrain and HSR. These detailed suggestions will be the subject of an upcoming post.

The rail infrastructure of the Downtown Extension and Transbay Transit Center is not nearly as sexy as sweeping glass towers and roof gardens: it's invisible, and for now, it's just a box. We can only wish that the architects realize the extent to which that simple, invisible box will determine the operational efficiency of San Francisco's train station for decades to come.