The Central Subway is an extension of the Muni Metro light rail system under construction in San Francisco, California, from the Caltrain commuter rail depot at 4th and King streets to Chinatown, with stops in South of Market (SoMa) and Union Square.

The subway is the second phase of the San Francisco Municipal Transportation Agency‘s Third Street Light Rail Project. The first phase opened to the public as the T Third Line in 2007. With the addition of the Central Subway, the T Third Line is projected to become the most heavily ridden line in the Muni Metro system by 2030.

The subway will serve major employment and population centers in San Francisco that are undeserved by rapid transit. SoMa is home to the headquarters of many of San Francisco’s major software and technology companies, and substantial residential growth is projected there. Union Square, located in the city’s downtown, is a primary commercial and economic district.Chinatown is the most densely populated neighborhood in the city. The Central Subway will connect these areas to communities in eastern San Francisco, including Mission Bay, Dogpatch, Bayview-Hunters Point and Visitacion Valley.

The budget to complete the Central Subway is $1.578 billion. The project is funded primarily through the Federal Transit Administration’s New Starts program. In October 2012, the FTA approved a Full Funding Grant Agreement, the federal commitment of funding through New Starts, for the Central Subway for a total amount of $942.2 million.The Central Subway is also funded by the State of California, the Metropolitan Transportation Commission, the San Francisco County Transportation Authority and the City and County of San Francisco.

Ground was broken for the Central Subway on February 9, 2010. As of February 2014, both tunnel boring machines (TBMs) had passed under Market Street and were tunneling under Stockton Stree toward Chinatown.Tunnel boring for the Central Subway was completed at Columbus and Powell Street in the North Beach neighborhood of San Francisco on June 16, 2014. This subway extension of the T Third Line is expected to open to the public by 2019.