The San Francisco Subway System is commonly referred to as BART, which stands for Bay Area Rapid Transit. BART is a rapid transit system that serves the San Francisco Bay Area in California. It connects the cities of San Francisco, Oakland, Berkeley, Fremont, and other surrounding cities. The system consists of six lines, with 44 stations in total. BART is the fifth busiest rapid transit system in the United States, and it is the most heavily used commuter rail system in the country.