From Wiki:
The Transbay Transit Center project was designed to include a tunnel (the Downtown Rail Extension or DTX) extending the terminus of the Caltrain commuter rail line from its current location at Fourth and King Streets; the downtown Caltrain extension is projected to alleviate roadway traffic and Caltrain rider delays, resulting in an estimated $20 million savings per year.[42] The Caltrain extension depends on the electrification of its rolling stock, as the current diesel engines are not appropriate for tunnel service. When this project is completed, Caltrain riders would no longer need to transfer to Muni in order to reach the downtown financial district. Additionally, the heavy rail portion of the terminal would be designed to accommodate the planned High Speed Rail from Los Angeles, which shares the right-of-way (Peninsula Corridor) with Caltrain between San Francisco and San Jose. BART has expressed interest in having their proposed Second Transbay Tube connect to the new terminal and Alameda.[43]

DTX was initially[when?] scheduled to be open to rail service in 2019 at a budgeted cost of US$2,600,000,000 (equivalent to $2,700,000,000 in 2017).[30] The DTX scope also includes moving the existing 4th & King Caltrain station underground.[44] Part of the DTX project also includes building out two below-grade levels below the TTC; one level would serve as the actual train platform, hosting six tracks and three platforms to accommodate Caltrain and HSR service; the other level would be a passenger waiting area, including ticket sales and retail amenities. The waiting area would be connected via tunnel to the BART/Muni Metro Embarcadero Station.[44]

As of August 2018, cost projections for the extension run as high as $6 billion, of which only $1 billion in funding sources has been identified. The project would not completed until 2029 at the earliest