Central Subway

Mom Chung tunnel boring machine

The Central Subway is the second phase of the Third Street Light Rail line, which opened in 2007. The Central Subway will extend this line northward from its current terminus at 4th and King Streets to a surface station south of Bryant Street and go underground at a portal under US 101. From there it will continue north to stations at Moscone Center, Union Square—where it will provide passenger connections to the Powell Street Station and BART—and at Chinatown, where the line will terminate. The Central Subway is expected to carry nearly 73,000 passengers a day, making it the second most utilized rail project in the Federal New Starts Program. On March 30, 2010 the Transportation Authority Board adopted a Baseline Budget, Schedule and Funding Plan for the project. Revenue service is scheduled for late 2018.

The total project budget is $1.58 billion, and the SFMTA has identified full funding from a combination of local, state, and federal sources. The primary funding source for the Central Subway is the federal New Starts program. To date, the project has received $137.9 million in funds programmed by the Transportation Authority and $177.4 million in federal New Starts funds. It also received $61.3 million in Prop 1A High Speed Rail connectivity funds.

On October 11, 2012, the (SFMTA) received the Central Subway project's Full Funding Grant Agreement (FFGA), from the Federal Transit Administration (FTA), the most significant funding-related milestone in the New Starts process The FFGA represents the FTA's official commitment of the $849 million in federal New Starts funds to the project, a major milestone that allowed the project to move forward into the main construction phase.

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Above: Mom Chung, one of two tunnel boring machines, will make its way beneath the city from 4th and Harrison to Stockton and Washington streets. Below: A visualization of the Chinatown Station, and the station area at Union Square. WATCH A VIDEO OF THE ASSEMBLY AND LAUNCH OF MOM CHUNG and one highlighting PROGRESS ON THE CENTRAL SUBWAY PROJECT DURING 2013

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Final Design was completed in October 2012. It was delivered under three contracts: DP1-Tunnels, DP2-Stations, and DP3-Systems. The project is being delivered in four construction packages, all of which have been awarded: Utility Relocation 1, Utility Relocation 2, Tunnels, and Stations and Systems.  Both utility relocation contracts have been completed and work is underway on the Tunnels contract and the Stations and Systems.

The Tunnels contract was awarded in June 2011, to the Joint Venture of Barnard/Impregilo/Healy. The $233.9 million contract consists of 1.5 miles of twin-bore tunnels underneath Fourth Street and Stockton Street, from I-80 to North Beach. Its major components include tunnel boring machine (TBM) procurement, construction of the TBM launch box and cross passages, construction of extraction shaft and portal, and headwalls for the Union Square and Moscone stations, among other elements. The contractor procured the TBMs from The Robbins Company, headquartered in Solon, Ohio.


Boring of the Tunnels commenced in June 2013. The contractor launched the first TBM “Mom Chung” on July 19. The machine has progressed seven and a half blocks and as of November 25, it is between Mission Street and Market Street. It is expected to cross under the Powell BART station on November 28. The second tunnel boring machine, “Big Alma,” was launched on October 24. Work has been completed on the headwalls for the Yerba Buena/Moscone and the street has been restored. Headwalls for the Union Square station were completed in mid-October. Mom Chung successfully crossed Market Street under the BART tube over Thanksgiving 2013; Big Alma followed in January 2014. Completion of the tunneling contract is scheduled for July 2014.

 In June 2013, the San Francisco Municipal Transportation Agency (SFMTA) gave notice-to-proceed to contractor Tutor Perini for the largest single contract ever awarded by the agency: the $840 million stations and systems contract. With a 27% DBE participation, the contract will construct the three stations and the overall systems for the project. As 2013 came to an end, the contractor had completed the demolition of the structures at the Yerba Buena/Moscone and Chinatown station sites in preparation for excavation of the station headhouses.Work continues on the implementation of the Pagoda Theatre option for the extraction of the TBMs.  Demolition of the Pagoda site in North beach was completed on September 24 and work on the shoring walls is underway. Since this location is outside of the approved project limits, the additional cost, estimated at just under $9 million, will be outside of the project's $1.578M baseline budget; MTA will utilize sources other than Prop K or FTA New Starts funding.

Project Goals

  • Extend the north-south corridor of MUNI's Metro System
  • Provide shorter travel times for passengers
  • Relieve congestion on the Stockton Street corridor
  • Provide connectivity to a future Geary line.


Download the latest Central Subway Fact Sheet.


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Watch this fascinating time-lapse video of the assembly and launch of Mom Chung, the first of two tunnel boring machines to begin its journey beneath the streets of San Francisco.


This video highlights the progress made on the Central Subway project in 2013, from launch box to North Beach.