The Vexed DTX Tunnel
Just a techie tidbit note today regarding the DTX tunnel that is at the heart of the engineering side of the dispute between CHSRA and TJPA on whether the current design meets CHSRA’s future needs. I know we’ve been over this too many times already, but somehow the issue needs to be put to bed such that everyone can save face and focus on maximizing California’s share of the $9.5 billion in federal dollars already on the table. Protracted tiffs don’t do much for the confidence of the general public, let alone potential private investors, in either TJPA or CHSRA.
My objective for this post is to suggest a tunnel sharing and operations timetable strategy that might help bridge the gap. First, a quick recap of the saga so far.
Issues Related to Planning and Funding
TJPA has been planning the entire Transbay Terminal since SF voters approved prop H in 1999, with the objective of bringing not just Caltrain but eventually also HSR under the same roof as a large number of bus services. The old building dates to 1939, has seen better days and is anyhow no longer up to seismic code. The only part that will be re-used are the bus ramps that once allowed electric trolley cars to run across the lower deck of the Bay Bridge. The underutilized neighborhood south of the new building will also be redeveloped.
The DTX tunnel is the bit between Caltrain’s existing terminus at 4th & King Street and the basement of the new terminal building. As designed, it features three tracks under Townsend and 2nd Streets and widens into a curved throat section to reach six platforms tracks accessed via an underground concourse level and three full-length wide island platforms. Of these, two will feature the level boarding height to be chosen by CHSRA and the other the one chosen by Caltrain. The outside tunnel tracks are to be shared by both services and are permanent as inbound and outbound tracks, respectively. The center track will apparently alternate between inbound and outbound traffic.
CHSRA has recently raised a red flag regarding the design, claiming it couldn’t support the 12 HSR trains per hour (tph, meaning that number each way) during peak periods claimed in the most recent ridership forecast for 2030. There seems to be fairly broad agreement among the readership of this blog that this is a new and excessive demand made public only after TJPA decided to bypass CHSRA in seeking a slice of the HSR dollars in the recent stimulus bill. Quentin Kopp is concerned this would set a precedent and cause CHSRA to lose control of project cost.
TJPA claims its plans are based on a previous estimate of just 4tph and a minimum dwell time of a full hour. The latter is itself a highly questionable requirement, given that terminal stations in Europe manage to turn HSR trains around in 6-10 minutes. They are treated as through stations that happen to require trains to reverse direction, generally with a change of driver. Old-fashioned terminus stations are grand buildings providing small town folk with access to a metropolis, reached after many hours or even days on the move. Thanks to high speed, even San Diego-SF-San Diego will amount to just a single eight-hour shift for employees, so in that sense SF could indeed be considered a through station along out-and-back routes originating in SoCal and Sacramento.
The Authority would be on much firmer ground if it stuck to AB3034, which point blank requires that the entire network must support headways of no less than 5 minutes. This refers to the time interval between the nose of one train and the next one running in the same direction on the same track. The technically required minimum is a function of speed and emergency braking distance at that speed. For moderate speeds, experienced operators of modern train control infrastructure can make do with as little as 2.5 minutes, so the legal target set by the bill is conservative. As will become evident shortly, the link between short headway capability and system throughput is not always straightforward.
Technical Issues and Solution Concepts
a) at just 500 feet, the two curves in the tunnel will prevent at least the HSR trains from running them at much more than bicycle speed. In addition, we expect screeching noises as the long wheelbase trucks needed for high speed stability are dragged kicking and screaming around the corner. Passengers on board and especially, anyone present on the platforms would hear a sound reminiscent of fingernails on a blackboard, just much louder. Special lubricants might help keep both the noise and the wear and tear down, but no brand-new design should need them. The radii are also too tight for off-the-shelf Japanese shinkansen designs, which need ~925 feet. Even for European designs, 500 is the number the marketing department made the engineers sign up to, not one they’d recommend for brand-new track.
b) the slow speeds combined with the tunnel layout combined with train lengths of 660-1320 feet mean that during peak periods, inbound and outbound trains of both operators will block each others’ paths to such an extent that the “throat” between the tunnel proper and the platform tracks becomes the throughput bottleneck. It’s highly unlikely that HSR could achieve 5 minute headways during Caltrain’s rush hour with that design.
Both post authors and commenters have propsed a wide range of solution concepts on both this blog and Clem’s. Here’s a recap:
- terminating some or all HSR trains at 4th & King
- a central rail station at Market & 7th instead of the Transbay Terminal
- redefining the Transbay Terminal to include a narrow two-level heavy rail station under Mission Street, reached via a short underground pedestrian passage and featuring moving walkways along the 1/2 mile concourse level
- keeping the tracks in the basement, but redesigning the DTX tunnel as a one-way single-track loop through the building from east to west; any future second transbay link would then be BART down Mission Street
- redesigning it to run up 3rd Street to increase curve radius for just two – possibly individual track – tunnels
- minimally, some tweaking of the curve radii and the use of curved switches in the throat of the official three-track tunnel design
Dedicated Single Tunnel Tracks
In the vein of this last concept, I’d like to add one more suggestion to the list. Since HSR and Caltrain are supposed to get dedicated platforms and platform tracks anyhow, I figured why not give each of them a single, dedicated track in the DTX tunnel as well. That would at least get the operators out of each others’ hair. A third track should be avoided to keep the tunnel engineering as straightforward as possible so the curve radii can be increased without breaking the bank.
The idea is really quite simple: each operator sets the signals in the tunnel track to outbound and runs a group trains out in quick succession. The timetable is arranged such that an equal number of inbound trains arrives at the mouth of the tunnel just as the last of the outbound trains clears it. The operator sets the signals in the opposite direction, the platforms tracks fill up again and the process repeats.
The advantage is that as long as traffic is guaranteed to be one-way, headways as small as three minutes are quite realistic. No outbound trains blocks any inbound one, no HSR train any Caltrain and vice versa. In other words, there is no throat in the classic sense, just two independent tunnel tracks that fan out to two resp. four platform tracks that are alternately used in one direction or the other for a well-defined period of time.
The disadvantage is that each operator’s timetable would feature a number of trains in quick succession, followed by a period without service in that direction. Note that in each of these groups of trains, the fastest service classes (e.g. express) come first, followed by the slower ones (e.g. semi-express). This ensures headways do not decrease below three minutes in nominal operations. For two consecutive trains of a slower class, it may go back and forth from the minimum value to a higher one.
To get a better sense of the kind of timetable this would produce, I made a number of assumptions and plugged them into a spreadsheet. In addition to the three-minute headways, I assumed that the cumulative delay for an outbound group of trains would be no more than one minute. For inbound groups, I allowed a three-minute buffer and assumed a two-minute dwell time for trains making a stop between San Jose and SF. Finally, I assumed Caltrain’s commuter EMU equipment would traverse the 1.3mi tunnel in two minutes (39mph average speed thanks to high acceleration and short wheelbase trucks), whereas HSR trains would take three (26mph average).
For service patterns based on half-length trains (660ft or less), the first inbound train would proceed to the end of the platform track. The second would stop before colliding with the first. However, since it’s a terminal station, the later arrival would have to be the first one to leave, resulting in uneven dwell times. Also, I assumed a platform track would be cleared entirely before trains from the next inbound batch are admitted. This is how an operator would use the facility during rush hour. At off-peak periods, the ends of the platform tracks can be used for parking.
The results were as follows:
- For HSR service based entirely on single trainsets, I assumed a group size of four trains occupying one track on each island platform. The first two were express, the second two semi-express trains on the SJ-SF segment. This affects the length of time the track needs to be reserved for inbound traffic. Note that I did not consider service class impacts south of San Jose for this preliminary analysis. Instead, I assumed appropriate service groups would be created via wait states in San Jose.
After allowing four trains to leave, four new inbound ones would be admitted, followed by the other four trains still in the station and four inbound ones to replace them. The period for this pattern of 8 trains each way works out to exactly 60 minutes, i.e. 8 tph. The minimum dwell times for the trains in each group worked out to 38, 44, 36 and 42 minutes, respectively. Because of the buffers built into the schedule, they could be up to four minutes longer.
- I also looked at HSR service based exclusively on full-length trains. With a group size of four, this also works out to 8 tph – but each of train would now have twice as many seats! The snag is that the minimum dwell times are down to just 11, 11, 9 and 9 minutes, respectively. As discussed above, that is considered enough for a terminal in Europe. However, if a large fraction of seats is actually occupied in SF, seat reservations and some pedestrian flow control would be highly advisable to avoid delays. Evidently, trainset utilization rates are much higher if full-length trains are used. Note that it’s very easy to couple and uncouple HSR trainsets at stations, so operators could transparently switch to single trainsets during off-peak hours.
- HSR service based on full-length trains and a group size of just two yielded a throughput capacity of 7.06 tph, with minimum dwell times of 21 minutes.
- Separately, I looked at Caltrain local service based on 8-car trains that would be used during rush hour. With a train group size of four, throughput worked out to a very respectable 9.23 tph. Minimum dwell times were 6, 12, 6 and 12 minutes, respectively.
Fiddling with the parameters, it quickly became apparent that the minimum headway had the greatest effect on throughput, followed by the time required to traverse the DTX tunnel, dwell time at through stations down the line. Doubling the cumulative buffer period allowed for inbound Caltrain groups from 3 to 6 minutes resulted in a loss of only 1 tph in throughput.
Note that CHSRA still needs a solution for overnight parking of additional trainsets, as the first ones of the day will take some time to arrive from Merced and LA. The old Brisbane yard near Bayshore Caltrain would do nicely and could double as a transshipment center for High Speed Cargo trainsets that ride piggyback on single passenger trainsets during off-peak hours. Toward the end of the day, selected passenger trains would terminate at Millbrae/SFO.
Provided that inbound trains are grouped neatly and operators are willing to accept that consecutive trains in the same direction may run anywhere from 3 minutes to 13, 17 or even 21 minutes apart, aggregate throughput of over 17 tph is possible with this concept. For reference, a single loop track serving all six platforms with a minimum headway of 3 minutes would support 20 tph and provide more flexibility if CHSRA and Caltrain decide on a common platform height after all.
Cost Containment Opportunity
Just for kicks, I also looked at the possibility of saving some money in phase I by extending the dedicated single track for HSR beyond 4th & King to Bayshore. Caltrain already has four short tunnels in that stretch, CHSRA intends to bore new single track tunnels to either side of that. The CHSRA web site gives a time of 13 minutes for Transbay Terminal to Millbrae/SFO, so I figured it would take about 9 out to Bayshore. This longer single-track section caused HSR throughput to decrease from from 8 to 5.71 tph (4.14 tph for full-length trains with a group size of two but minimum dwell times of 33 minutes). For the single-trainset scenario with a group size of 4, minimum dwell times went up by 12 minutes. For full-length trains with the same group size and throughput number, they were unchanged.
In other words, it would be possible to shift some tunneling overheads south of 4th & King from phase I to phase II of the overall bullet train project, given that 4.14-5.71 tph will be enough for HSR operations for a while. Note that running northbound HSR trains east of Caltrain in that section would currently force UPRR trains to cross the HSR track. By the time HSR needs dual tracks between Bayshore and 4th & King, it’s entirely possible the mighty Port of SF will no longer be served by UPRR’s South City Switcher – a freight train running in streetcar mode.