BART stands for Bay Area Rapid Transit and is a public heavy rail transportation system serving the San Francisco Bay Area. The system includes four routes which total length is 104 ml or 164 km. There is also an additional route to Oakland Airport.

The system serves about 126 million passengers yearly and thus, is number 5 busiest heavy rail rapid transit system in the whole country.

Interestingly, the name of the system is often referred to as an acronym Bart, rather than an abbreviation BART.

More Info: en.wikipedia.org