BART is operated by the San Francisco Bay Area Rapid Transit District, a special-purpose transit district that was formed in 1957 to cover San Francisco, Alameda County, and Contra Costa County. The name BART is pronounced as a word, not as individual letters. In some ways, BART is the successor to the Key System, which ran streetcars throughout the East Bay and across the lower deck of the San Francisco -- Oakland Bay Bridge until 1958.

Some of the Bay Area Rapid Transit System's current coverage area was once served by the electrified streetcar and suburban train system called the Key System. This early 20th-century system once had regular trans-bay traffic across the lower deck of the Bay Bridge. By the mid-1950s that system had been dismantled in favor of highway travel using automobiles and buses, given the explosive growth of expressway construction. A new rapid-transit system was proposed to take the place of the Key System during the late 1940s, and formal planning for it began in the 1950s. Some funding was secured for the BART system in 1959, and construction began a few years later. Passenger service began on September 11, 1972, initially just between MacArthur and Fremont.

The new BART system was hailed by some authorities as a major step forward in subway technology, though questions were asked concerning the safety of the system and the huge expenditures necessary for the construction of the network. All nine Bay Area counties were involved in the planning and envisioned to be connected by BART.

The extension of BART into Marin was estimated to be as late as 30 years after the opening of the basic system. Initially, a lower level under the Golden Gate Bridge was the preferred route. In 1970, the Golden Gate Transportation Facilities Plan considered rapid transit links to Marin County via a tunnel under the Golden Gate or a new bridge parallel to the Richmond-San Rafael Bridge but neither of these plans was pursued...

In addition to San Francisco, Alameda, and Contra Costa Counties, Santa Clara County, San Mateo County, and Marin County were initially intended to be part of the system. Before construction started, Santa Clara County Supervisors opted out in 1957, preferring instead to build expressways. In 1961, San Mateo County supervisors voted to leave BART, saying their voters would be paying taxes to carry mainly Santa Clara County residents. Although Marin County originally voted in favor of BART participation at the 88% level, the district-wide tax base was weakened by the withdrawal of San Mateo County. Marin County was forced to withdraw in early 1962 because its marginal tax base could not adequately absorb its share of BART's projected cost. Another important factor in Marin's withdrawal was an engineering controversy over the feasibility of carrying trains across the Golden Gate Bridge.

Bay Area Rapid Transit (BART) is a rapid transit system serving the San Francisco Bay Area. The heavy-rail public transit and subway system connects San Francisco with cities in the East Bay and suburbs in northern San Mateo County. BART operates five lines on 104 miles (167 km) of track with 44 stations in four counties. With an average weekday ridership of 379,300 passengers, and 309,420 weekend day passengers, BART is the fifth-busiest heavy rail rapid transit system in the United States.