"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.
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."