The Caltrain commuter rail system links San Francisco with San Jose / Gilroy and the cities in between. Originally built in 1863 by Southern Pacific, today the system offers local and express service between the numerous residential and commercial areas in the SF Peninsula and Silicon Valley.

Like BART, Caltrain has undergone astronomical ridership growth in recent years. With 60,000 daily riders, Caltrain’s single line ranks among the busiest rail corridors in the US, and a ridership of 110,000 is anticipated by 2050. To meet this target, Caltrain will undergo the most significant capital construction in its history over the next 15 years. The previous article of this series discussed how Caltrain plans to to run more frequent and faster service along the SF peninsula by 2021.

After electrification, Caltrain has several expansion plans in the work. This article will specifically center on a medium-term plan to extend Caltrain past its current Mission Bay terminus into the heart of SF’s Financial District via an underground tunnel, the Downtown Tunnel Extension (DTX). Future electric multiple units (EMUs) will enable Caltrain service to run in the underground tunnel (underground diesel trains require expensive ventilation systems to mitigate smoke hazards).

This article discusses a broad overview of potential alignments for the DTX. For more technical analysis on the DTX and potential challenges and solutions, check out the Caltrain-HSR compatibility blog.

Alignment: Rincon

Caltrain currently terminates at the 4th and King intersection in the South of Market (SoMA) neighborhood of SF, which is a considerable distance from SF’s main thoroughfare, Market Street, and several important commercial districts: Civic Center, Union Square, and the Financial District, to name a few. Direct service to the Financial District represents Caltrain’s biggest opportunity of growth; the walkshed of a station there is estimated to have more jobs than at every other station in the system combined.

While the Central Subway will connect SoMa with Union Square and Chinatown by 2020, SF’s main employment district, the Financial District, remains only accessible from Caltrain through a transfer to SF Muni’s N Judah service. With further growth expected in the SoMa region, the current influx of Caltrain commuters transferring to and from Muni at 4th and King will likely cause unmanageable congestion. Thus, a rail extension there will likely be needed in the near future.

Fortunately, there exists a suitable site for a station in the Financial District. Currently, the Transbay Transit Center (TTC) is under construction at the Rincon Hill location of the former Transbay Terminal, which was served by the Key System and buses coming into SF. The TTC is envisioned as a regional transit hub for SF to the rest of the Bay Area: its upper levels will serve buses, while its underground levels will host a train station.

The extension has been under planning since the 1990s, but plans have only moved forward due to the ongoing planning of the California High Speed Rail (CAHSR) system. Like the DTX, CAHSR has been under planning since the 1990s. The passage of Prop 1A in 2008 has paved the way for a sub-3 hour train service between LA and SF. Because Prop 1A specifically mandates train service to SF’s TTC, train service there will become a reality within the next 15 years, provided that CAHSR construction isn’t abruptly cancelled due to fiscal issues.

The TTC’s location presents some interesting and important technical challenges. The most prominent is the inherent capacity constraints of such a terminal. Only three platforms serving six tracks will be built, and arriving trains must quickly reverse and depart back down the peninsula. (Such a constraint can be relieved by a further extension that will be discussed in a future post, but such a plan would be long-term, i.e. after 2030.)

Moreover, the approach tracks to the station poses another capacity constraint. As the diagram below demonstrates, the current alignment for the DTX, hereby referred to as the Rincon Hill alignment, has two turns between the TTC and the current 4th & King terminus. Trains must inherently slow down to traverse these sharp curves, which further reduces thoroughput and capacity.

As the DTX will not be under construction for at least five years, planners should carefully examine and determine an optimal route between Mission Bay and Downtown SF. Brian Stokle of Urban Life Signs has illustrated several potential alignments with their pros and cons. These routes fall in one of two categories, both of which will be discussed below.

Alignment: SoMa

This option uses only one curve, rather than three sharp turns, to take a more direct and quicker route into the TTC under Howard St compared to the Rincon alignment. This alignment introduces two new underground stops:

Mission Bay (7th St / 16th St or 6th & King): Presumably a replacement for the existing 22nd St Potrero Hill stop, this station serves a major transit corridor. This stop would be near UCSF Mission Bay and the planned future Warriors arena.

Moscone Center: The station lies near SF’s commercial center at Union Square. The station would offer connections to Muni’s Central Subway, and potentially an underground passageway connection to Powell St BART.

The main advantage of this alignment would be the new service to the neighborhood around the Powell St BART station. Moreover, a potential underground connection to Powell St would reduce congestion at the planned transfer point between the TTC and Montgomery St / Embarcadero BART stations.

Nonetheless, several important tradeoffs exist that may make the route politically infeasible. The current 4th and King terminus would be removed and not served anymore, potentially disrupting the existing commutes of passengers. Moreover, the DTX would likely travel under several private land and buildings, and the tunnel may be technically tricky to construct, given the frequency of earthquakes in SF. To reduce the amount of private properties above the tunnel, a variant of this route would run under Mission St, with a stop at 5th St rather than Moscone Center.

Alignment: UCSF Mission Bay

This alignment retains the existing approaching into the TTC under Second St, but reroutes the DTX under 4th St between King St and 20th St. This alignment introduces two new underground stops:

UCSF Mission Bay (4th St / 16th St): Presumably a replacement for the existing 22nd St Potrero Hill stop, this station serves a major transit corridor. This stop directly serves UCSF Mission Bay and the planned future Warriors arena.

3rd St and King: This replacement station for the existing 4th St and King terminus would serve the same destinations (e.g. AT&T Park) and maintain connections to Muni’s N and T lines.

The northern half of this alignment would be unchanged from the Rincon Alignment and thus would not have to be restudied, potentially saving time and cost. Like the SoMa alignment, trains will travel at higher speeds in a slightly straighter tunnel, reducing travel times and improving capacity and operation flexibility. The UCSF alignment also requires short sections under private property, which may present a political challenge. Compared to the SoMa alignment, the UCSF alignment retains service to existing neighborhoods, but does not introduce new service into the commercial heart of SF, i.e. Union Square / Moscone Center.

Conclusions

The current Rincon DTX alignment will likely cause capacity and speed constraints due to the presence of several sharp curves. Two proposed alternative alignments would reduce these constraints and decrease travel times to the proposed Downtown SF terminal at the TTC. Both routes have their pros and cons, but the tradeoff can be summarized as follows: it is impossible for Caltrain to serve both the Union Square and Mission Bay neighborhoods. A more detailed analysis, preferably involving ridership trends and census data, should be conducted to determine the most cost-effective alignment for Caltrain’s current passengers.

The DTX is Caltrain’s most prominent extension under planning, but there exist other routes under consideration. The next article of this series will examine Caltrain’s potential involvement in two Transbay crossings under consideration. These Transbay crossings will create transfers between Caltrain and BART, which will be crucial to the development of the Bay Area’s integrated rail transit system.

(Final note: The author of this article supports the UCSF Mission Bay alignment. While there would be no Caltrain stop near Powell St BART, passengers still have a relatively convenient transfer at the proposed 3rd and King stop to Muni’s Central Subway, which has stations at both Moscone Center and Union Square. On the other hand, the SoMa alignment eschews the China Basin neighborhood entirely, with a very indirect transfer provided at the Moscone Center MUNI stop. Potential concerns regarding congestion between Embarcadero and the TTC can be addressed with a new intermodal stop in Oakland between BART and Caltrain, which will be discussed in the next article of this series.)