"Right now there's a gap of about four hours between when the last trains of the day leave, usually around midnight, and when the first trains of the day start up, around 4 a.m. on weekdays. (Those are end-of-the-line times; last-train times from individual stations may be later. For example, the latest SFO to Pittsburg/Bay Point train on a Saturday night leaves Embarcadero Station at 25 minutes past midnight, actually Sunday morning. Check the schedule
That short window of time without service is used for essential nightly track maintenance. Unlike some public transit systems with multiple sets of tracks on the same routes, BART doesn't have the duplication that would allow us to run trains on one set while performing maintenance on another. Third-rail power has to be shut down for maintenance crews to be able to operate safely and do the work that keeps the system safe and reliable. And the trains can’t run when the power is down.
A little history: BART was never intended to be a 24-hour system. When cost projections were initially developed, the residents of the region who voted to approve BART supported a system that would have limited hours of operation. (In its early days, BART was even closed on weekends.) You can read more about BART's history in the History and Facts
section of our website. BART does extend service for certain special occasions, such as New Year's Eve celebrations and some sporting events, when large late-night crowds are expected."