Known for its moderate weather, diverse demographics, and thriving tech sector, the SF Bay Area is the fifth-largest metropolitan region of the US. The Bay Area is currently home to 7.5 million people, with 9 million expected by 2050.

The Bay Area is divided into nine counties. While each county of the Bay Area is served by its own transit system, the region is also served by two major regional rail systems.

Bay Area Rapid Transit (BART) has connected the East Bay with San Francisco and the Peninsula for over 40 years (history summarized here). BART is essentially a commuter rail system using subway technology, with four branch lines in the East Bay feeding into one major trunk line in SF and San Mateo County.

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.

In light of unprecedented ridership growth, both BART and Caltrain are undergoing improvements to increase capacity. The previous two series on this blog discussed the medium-term road maps for both BART and Caltrain, which are summarized in the table below.

System BART Caltrain 2020 Open new extensions in Santa Clara and Contra Costa Counties New train cars with extra doors to relieve crowding Electrify the SF – SJ corridor New EMUs to provide faster and frequent service 2030 Upgrade the power substation and signaling systems to increase capacity Build crossovers to better serve the urban sections of the system Build infill stations to increase ridership in denser urban areas Build four tracks between San Mateo and Redwood City to increase express train service Extension into Downtown SF Transbay Transit Center Prepare an integrated timetable for future HSR service

Building on the topics presented in the previous two series on BART and Caltrain, this article will discuss proposals to integrate the two systems. In particular, these proposals center on two new SF Bay crossings, the Dumbarton Rail Corridor and the Second Transbay Tube, to increase overall transportation capacity in the region. By doing so, the SF Bay Area will have a more seamless regional rail system with intersystem transfers to better serve its residents over the next decades.

Overview

The SF Bay Area’s unique geography presents numerous challenges for civic engineers. Due to the presence of the SF Bay, the denser regions of the East Bay and the West Bay (SF and the Peninsula) have very limited transportation links. To travel across the Bay, travellers currently must use one of only four connections:

SF – Oakland Bay Bridge: 10 lanes of capacity for cars and buses

Transbay Tube: 2 tracks of capacity for BART

San Mateo – Hayward Bridge: 6 lanes of capacity for cars and buses

Dumbarton Bridge (Menlo Park – Newark / Fremont): 4 lanes of capacity for cars and buses

All four crossings are at capacity during rush hour, and heavy traffic plagues several of them during the midday. Thus, the Bay Area should plan for new Transbay crossings to relieve projected crowding. The two primary candidates include the Dumbarton Rail Corridor and the Second Transbay Tube.

Dumbarton Rail Crossing

Three major corridors serve traffic between the East Bay and the Peninsula cities south of SF: Highways 92, 84, and 237. All three corridors are constantly congested in the peak commute direction daily. Thus, an additional rail corridor could provide faster travel across the Bay, as well as additional capacity and relief.

The Dumbarton Rail Corridor Project aims to extend current Caltrain rail service from the Redwood City Junction to Union City BART, with additional stops at the existing Fremont / Centerville station, Newark, and East Palo Alto (Facebook HQ). The project would restore two existing railroad bridges (310 and 188 feet long) currently rendered inactive by a 1996 fire, and projected cost is a factor less than similar projects such as the BART extension to Silicon Valley. In the long term, the corridor could also potentially be extended via a tunnel to Pleasanton for usage by Altamont Corridor Express trains as well.

While funds for the project were diverted during the 2008 recession, Facebook-driven development and subsequent traffic in East Palo Alto has renewed interest. At the moment, potential construction of the project would occur in two phases, with the rail bridge restoration deferred to Phase 2. In the meantime, the City of Menlo Park has proposed the first phase, a new service between Redwood City and East Palo Alto.

Full restoration of the Dumbarton Rail Corridor would add four new stations to Caltrain’s system:

East Palo Alto (Willow Ave), serving Facebook

Newark (Cherry St)

Fremont – Centerville

Union City (BART)

The Dumbarton Rail corridor would also enable future expansion north towards Oakland, or east towards Pleasanton along the Altamont Corridor.

Second Transbay Tube – BART?

The more ambitious plan involves the construction of a second underwater rail tube between SF and Oakland. With an additional platform at Embarcadero Station and signal upgrades, the existing BART Transbay Tube can likely support 30 trains per hour (one every 2 minutes), representing a capacity increase of more than 25 percent. However, transbay rail patronage has exploded in recent years due to cultural shifts away from automobile traffic, and at some point, a new crossing will be needed.

Regional planners have studied a Second Transbay Tube, hereafter abbreviated STT, since the 1990s. The most recent development came in late 2014, when BART formally announced plans to study not just the Second Transbay Tube, but also additional subway service in SF, primarily along Geary Blvd. The BART line would eventually turn south and rejoin the main BART line at Daly City.

While the alignment under 19th Ave and Geary Blvd is generally accepted by transit planners and enthusiasts, there is ongoing discussion regarding the exact route between Van Ness / Geary and Oakland. Downtown SF and SoMa have several important destinations that could be served, but it is unlikely a single alignment can serve all of them:

Civic Center (BART, Muni)

Union Square (BART, Muni)

Moscone Center (BART, Muni)

Transbay Transit Center – Financial District (Caltrain, CAHSR)

AT&T Park – China Basin (Caltrain, Muni)

UCSF Mission Bay / Warriors Arena (Muni)

On the other side of the Bay, the STT would likely connect to a subway under Alameda Island. In Oakland, new BART stops would be constructed at Alameda Point and Jack London Square. The Alameda subway could connect to the Fremont Line at the new San Antonio infill station or to the Richmond and Pittsburg / Bay Point lines at Macarthur station. New BART service could potentially be introduced in the I-980, San Pablo Ave, and/or Eastmont corridors.

Such an ambitious plan will likely to take several decades to implement, however. The main impediment to such a plan, of course, is cost and time. A full BART line from Daly City to Oakland via 19th Ave, Geary Blvd, Third St, and the STT would likely cost $10 to $20 billion, making it the most expensive non-HSR rail construction in the US. Constructing the new BART line going under multiple rail tunnels in Downtown SF and Oakland would introduce numerous technical challenges. Additionally, integrating the new tunnel to the existing BART system could be difficult; in particular, connecting the Alameda subway with existing lines in Oakland may introduce convoluted service plans with lots of interlining among routes.

Second Transbay Tube – Caltrain?

Perhaps a further extension of BART isn’t the correct solution. Due to its unique track gauge, BART is unable to leverage existing rail infrastructure in the East Bay, raising construction costs. BART’s unique specifications lead not only to expensive construction costs, but also prevents compatibility with existing standard-gauge intercity rail to the Central Valley and Southern California.

Alternatively, Caltrain, which will be extended to the TTC in the next 20 years, could also be tasked with providing additional transbay service. Once the DTX tunnel to the TTC is built, a new tube for Caltrain and HSR across the Bay should only cost an addition $5 – 10 billion at most, though it should be noted that US transportation projects are infamous for cost overruns. Importantly, Caltrain’s use of standard-gauge rail would also enable direct connections to the existing Amtrak corridor in the East Bay. Subsequently, existing Amtrak ROWs could be electrified and utilized at relatively little cost. As a side benefit, Bay Area regional commuter trains would reverse at outlying East Bay terminals rather than the high-traffic TTC, which will improve operating efficiency.

New service would permit faster service between the Peninsula and the East Bay, with several new or underserved origin-destination pairs introduced.

STT Proposal: Third St (SF), Eastshore, and Alameda Corridors

After examining all the above options, this article proposes that the STT be an extension of future Caltrain services. The STT would serve the Eastshore and Alameda County corridors, with a provision for direct SF – Sacramento service in the future.

In SF proper, Caltrain would be rerouted in a tunnel along Third St between 20th and Howard Sts. Four new stations would be built north of Bayshore as follows:

Oakdale Avenue, serving Bayview

16th St – Mission Bay (replacing 22nd St), serving UCSF and the future Chase Center

Townsend St – South Beach (replacing 4th and King), serving SoMa and AT&T Park

Transbay Transit Center

After crossing the San Francisco Bay in the STT, the line would be split into two branches, the Alameda County and East Shore Corridors.

The Alameda County corridor would head east from the STT towards Oakland, with stations at

Oakland – Jack London Square (underground), with transit connections to Alameda Island and Downtown Oakland.

Fruitvale Ave, connection available to BART

Oakland Coliseum (existing), transfer available to BART

Future service could be extended down to Fremont, where trains can continue further to San Jose or across the Dumbarton crossing.

The Eastshore corridor would head north from the STT north along the current Capitol Corridor route, with stations at

West Oakland – 7th St, transfer available to a (future) infill BART station

Emeryville (existing)

Berkeley – University Ave (existing)

Albany – University Village

Richmond (existing, transfer available to BART

San Pablo

Bayview

Hercules, ferry connection

Initial electrification would end at the Hercules Transit Center, with potential electric service to Sacramento in the long term.

The most noticeable corridor not served under this plan, Geary Blvd, would instead be served by a future Muni Metro line that would be more cost-effective.

Conclusions

To accommodate expected growth and redevelopment, the SF Bay Area will need to undergo several rail expansion projects in the near future. Most center on BART, but from a cost and operations standpoint, it is likely that Caltrain extensions along existing rail corridors will likely be more cost-effective. Caltrain extensions along the Dumbarton corridor and the STT, with transfers to BART in the East Bay, would create a more unified and integrated rail transit system for the Bay Area.