Central Subway: Alternative Alignments

This is Part 2 of a five post series on the Central Subway project. Click here to navigate the table of contents for these posts.

Studies carried out for the past several years gave rise to a few different alignments of the Central Subway. Although the SFMTA now prefers a plan that would tunnel under Fourth and Stockton Streets, the alternatives are all still officially being considered and studied. This post will describe each of the alignments. Each description is accompanied by a map; for a full sized map, please click through the image links.

Alternative 1: This is the standard no-project alternative, used for comparison purposes. Under this plan, the T-Third initial operating segment from 4th/King to Visitacion Valley is the only capital improvement, but additional improvements would be made to bus service. Depending on the time and day, headways on the short line 30-Stockton would be 3-10 minutes; the 30 long line headways would be 7.5-10 minutes. Headways would also be reduced on the 45-Union/Stockton, except on Sunday evenings. More notably, though, the 22-Fillmore would be extended along 16th Street to serve Mission Bay, and the 45-Union/Stockton would be extended through Mission Bay to a terminal point at 3rd and 20th Streets. Both trolley bus lines would thus connect to the T-Third.

The project alternatives are:

Alternative 2 (Enhanced EIS/EIR): Alternative 2 contains the changes made to the original FEIS/FEIR plan from 1998 and is the most convoluted of the three project alternatives. T-Third trains bound for Chinatown would pass through the existing T station at 4th/King, turn right onto King Street as they do now, and then turn left on 3rd Street, stopping at a new surface station at 3rd/King. (This station would be close to, but different from, the existing ballpark station on King between 2nd and 3rd.) After stopping at 3rd/King, trains would travel northward on 3rd in a semi-exclusive right-of-way and enter the single-track subway portal between Bryant and Brannan. In the southbound direction, trains traveling south in the tunnel under 3rd Street would swing west under Harrison and then continue south on 4th Street, exiting the subway at a single-track portal on 4th, between Bryant and Brannan. Trains would stop at the existing 4th/King station and then proceed southward as they do now.

Northbound and southbound tunnels would join up at 3rd/Harrison in a stacked configuration, with the northbound track on top of the southbound. This creates a two-level platform configuration for Moscone Station, located on 3rd between Howard and Folsom, with station entry points on Tehama. North of Moscone, tracks would transition to a side by side setup to allow a shallow crossing above the existing BART/Muni tunnel under Market. The Market Street Station would be located on 3rd between Market and Mission and would feature a 440-foot underground pedestrian walkway to Montgomery BART/Muni Station. After crossing Market, the tunnel would veer west onto Geary, returning to a stacked track configuration, and then veer once more onto Stockton. The Union Square Station would be centered on Post/Stockton and would have a two-level platform setup similar to Moscone Station. Station entrances would be located in Union Square and on Stockton. After Union Square, the stacked tunnel configuration would return once more to a side-by-side configuration as trains travel north under Stockton, terminating at Chinatown Station, on Stockton between Clay and Sacramento, accessible via an off-street station entrance on the first floor of a new building on Stockton. The station would have two side platforms, as well as the necessary storage and crossover track extending as far as Jackson. Here is a map of the Alternative 2 alignment:

Both versions of Alternative 3 (options 3A and 3B) shift the the South of Market part of the alignment off 3rd Street and onto 4th Street, permitting one turn of the tunnel to align underneath Stockton Street, as opposed to the more convoluted route in Alternative 2. Both 3A and 3B share a side-by-side double track configuration for the entire length of the project.

Alternative 3A (Locally Preferred Alternative): Alternative 3A was developed in response to the increasing costs of Alternative 2. A semi-exclusive right-of-way would extend north of the existing 4th/King station, and trains would enter and exit the subway via a double-track portal between Townsend and Brannan, just north of the Caltrain Depot. Traveling north, the first subway station is Moscone, which is now located on 4th between Howard and Folsom. The main entrance to Moscone Station would be an off-street entrance, the first floor of a new building; the site currently houses a gas station. Other station entrances would be located on the north side of the station, in front of Moscone Center West. North of Moscone Station, the tracks continue side-by-side but descend, to prepare for a deep tunneling underneath the current BART/Muni subway. The next station is a combined Union Square/Market Street Station, centered on O’Farrell/Stockton. Northern entry points are in Union Square and along Stockton; on the south side, the station would be accessible via current entry points to the Stockton/Ellis corner of the Powell BART/Muni station. After Union Square, trains continue north to Chinatown Station, in the same location as in Alternative 2. There is also an option to extend the construction tunnels into North Beach to extract the tunnel boring machines. The TBMs would emerge in the middle lanes of Columbus Avenue, between Union and Filbert, right next to Washington Square. This option would add $54 million to the cost of the project. Here is a map of Alternative 3A including the optional tunnel to North Beach:

Alternative 3B (Modified Locally Preferred Alternative): Alternative 3B is a revised version of Alternative 3A, designed to further cut costs from 3A. The tunnel is still aligned underneath 4th and Stockton Streets, but the portal and station configurations are different. Although 3B does respond to certain problems in 3A, station platform lengths were reduced from 250 feet to 200 feet, which is long enough to accommodate two-car trains.

Notably, 3B adds an additional surface station at 4th/Brannan, providing service to to a growing section of South of Market that was skipped over by Alternative 3A (in which trains did not stop between 4th/King and Moscone Stations). The 4th/King and 4th/Brannan stations could be connected either via semi-exclusive right-of-way, or possibly mixed flow operations with private autos. Mixed flow would add a little more parking and permit a landscaped median but would also force trains to compete with autos for street space. Under either plan, 4th Street would be one-way southbound only north of Bryant, returning to two-way operation between Bryant and King.

With the introduction of a new surface station at Brannan, the subway portal shifts northward to the block between Harrison and Bryant, underneath Interstate 80. (Recall that this location is right next the storage facility that will be used to temporarily store Golden Gate Transit buses during construction of the Transbay Transit Center, a project concurrent with the Central Subway.) North of the portal, Alternative 3B is very much like 3A, only with shorter station platforms. The major difference is that the terminal Chinatown Station is shifted slightly north, centered on Washington Street, a bit closer to the center of Chinatown. The option to extract the TBMs in North Beach is the same as described in 3A. Here is a map of Alternative 3B:

Although Alternative 3B is currently the preferred plan, all three alignments are technically under consideration until one is finally chosen.

Alignment maps courtesy San Francisco Municipal Transportation Agency.