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.