An extensive rail infrastructure that provides a mix of services exists within the nine Bay Area counties. Bay Area Rapid Transit, commonly known as BART, provides subway service in San Francisco and parts of Oakland and Berkeley, and commuter rail service to Contra Costa County, Alameda County, San Mateo County, and Santa Clara County. Caltrain, which runs on the right-of-way of the historic Southern Pacific Railroad, provides commuter rail service on the San Francisco Peninsula, linking the cities of San Francisco, San Jose, Gilroy, and numerous peninsula cities in between. The Millbrae Intermodal Terminal provides transfers between Caltrain and BART. The Altamont Corridor Express, commonly known as ACE, also provides commuter rail service, but from the Central Valley into Silicon Valley, terminating in the San Jose Diridon Station.