A tunnel has been proposed to extend Caltrain from its north end in San Francisco at 4th and King to the newly built Transbay Transit Center, closer to the job center of San Francisco and BART, Muni, Transbay AC Transit buses, and long-distance buses. As of 2012 only the structural "train box" below the Transbay Terminal has been funded and is being built. In April 2012 the Metropolitan Transportation Commission decided to make the remainder of the $2.5 billion extension its top priority for federal funding. The extension would also serve the California High-Speed Rail system.