The Downtown Rail Extension (DTX) is a planned second phase of the San Francisco Transbay Transit Center (TTC). When complete, it will extend the Caltrain Peninsula Corridor commuter rail line from its current northern terminus at 4th and King via a tunnel. The new terminus will be near the Financial District and will provide intermodal connections to BART, Muni, Transbay AC Transit buses, and long-distance buses. In addition, the California High Speed Rail Authority (CHSRA) plans to use DTX and the Caltrain-owned Peninsula Corridor for service on the CHSRA San Francisco–San Jose segment. Because DTX uses a long tunnel, current diesel locomotives are not suitable and the Caltrain Modernization Project (CalMod), which includes electrification of the line and acquisition of electrified rolling stock, is a prerequisite. Estimated at $6 billion, it is projected to cost more than the entire first phase of the Transit Center. Only the "train box", the structural shell surrounding the lowest level of the TTC, has been funded as part of Phase 1 construction. , full funding has not been obtained for the entire Phase 2 project. The Transbay Joint Powers Authority (TJPA) is seeking funding from various federal (Federal Transit Administration New Starts grants), state (SB1, the CAHSR project), regional, and local sources. From about 2013 to 2018, the final route between the current 4th and King terminus and the TTC was uncertain. Former San Francisco Mayor Ed Lee had proposed an alternative route in 2015 which would bypass 4th and King, extending Caltrain and high-speed rail to the Transbay Terminal through a new tunnel branching from the existing line at the 22nd Street station, then following a route generally under Third Street to TTC. In 2018, the San Francisco Planning department announced the preferred final route: a new tunnel under Pennsylvania Avenue that would connect to the originally designed DTX route from 4th and King to the TTC. In addition, the TTC is a candidate for the San Francisco terminus of a second Transbay Tube between San Francisco and Alameda Island, which would add direct BART service. Stations and service patterns The design of DTX includes, as of the 2015 environmental impact report, two new below-grade stations: 4th and Townsend: an underground station adjacent to the current 4th and King Caltrain terminal Transbay Transit Center: a station on the lowest level of Transbay Transit Center that opened in August 2018. Both stations would be