Bay Area Rapid Transit, mostly known as BART, is a rapid transit public transportation system serving the San Francisco Bay Area in California. The heavy rail elevated and subway system connects San Francisco and Oakland with urban and suburban areas in Alameda, Contra Costa, and San Mateo counties. BART connects 48 stations along six routes on 112 miles (180 km) of rapid transit lines, including a ten-mile spur line in eastern Contra Costa County which utilizes diesel multiple-unit trains and a 3.2-mile (5.1 km) automated guideway transit line to the Oakland International Airport.