"The Transbay Joint Powers Authority (TJPA) has primary jurisdiction with respect to all matters concerning the financing, design, development, construction, and operation of the Transbay Program. The TJPA is a joint exercise of powers authority created by the City and County of San Francisco, the Alameda-Contra Costa Transit District, the Peninsula Corridor Joint Powers Board, the California High Speed Rail Authority, and Caltrans (ex officio). The TJPA is managed by TJPA staff and is overseen by an eight-member Board of Directors.

Phase 2 of the Transbay Transit Center Program includes construction of the Downtown Rail Extension, including a new Fourth and Townsend Street Caltrain station; completion of the Transit Center’s train station, including a pedestrian connection to BART and Muni; and a new intercity bus facility.
The Downtown Rail Extension

The Downtown Rail Extension (DTX) will extend Caltrain commuter rail from its current terminus at Fourth and King streets to the new Transit Center. It will also deliver the California High-Speed Rail Authority’s future high-speed rail service to the Transit Center.

The 1.3-mile rail extension (1.95 miles of total construction length) will be constructed principally below grade using cut-and-cover and mined tunneling methods underneath Townsend and Second Streets. The project includes an underground station at Fourth and Townsend streets, six structures for emergency exit, and ventilation along the alignment, utility relocation and rail systems work."

CAHSR will use the tunnel when it is ready. If it gets to San Francisco before then it will use 4th and Townsend.