Bay Area Rapid Transit (BART) is a heavy rail rapid transit system in the San Francisco Bay Area. It is administered by the San Francisco Bay Area Rapid Transit District government agency in three California counties: Alameda, Contra Costa, and San Francisco. BART is the fifth busiest rapid transit system in the United States, averaging between 433,000 and 455,000 weekday passengers by 2015.
Mondays through Saturdays, BART trains run on five routes; four are transbay routes connecting San Francisco to Oakland and various destinations in the East Bay, while the fifth, the Richmond–Warm Springs line, runs exclusively in the East Bay. Two of the five routes do not run on nights and weekends, and one is truncated on Saturdays, but all stations remain accessible by transfers via other routes. In September 1972 BART's first route opened: Fremont to MacArthur, extended to Richmond in January 1973. Concord to MacArthur started in May 1973 and Montgomery Street to Daly City began in November 1973. The original system was completed in September 1974 when trains first carried passengers through the underwater Transbay Tube. BART's three routes then were Concord–Daly City, Fremont-Daly City and Richmond–Fremont.
The Concord-Daly City line was extended to North Concord/Martinez in 1995 and to Colma and Pittsburg/Bay Point in 1996. BART's fifth route, the Dublin/Pleasanton–Daly City line, began when the branch to Dublin/Pleasanton opened in 1997. The San Mateo County line was extended south from Colma to San Francisco International Airport and Millbrae in 2003. BART passengers can reach Oakland International Airport on BART's new automated guideway transit (AGT) system, the Coliseum–Oakland International Airport line.