The Central Subway, Third Street Light Rail Phase Two Project extends the existing SFMTA Light Rail System T-Line service from Fourth and King streets, north along and under Fourth Street to Market Street, under the BART and Muni Metro tunnels adjacent to Powell Station, and then north along and under Stockton Street to Chinatown Station. The work includes construction of three underground stations at Chinatown, Union Square/Market Street and Yerba Buena/Moscone areas with one surface station at Fourth and Brannan Street in addition to the construction of the systems and track through the tunnels and in the stations.

When the Central Subway is completed, Third Line trains will travel mostly underground from the 4th Street Caltrain Station to Chinatown, bypassing heavy traffic on congested 4th Street and Stockton Street.