|Early mockup of Transbay layout|
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.
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.
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|
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.
|Ideal route locking table, weighted by traffic|
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:
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|
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|
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
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.
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.