About 20-30 years ago, when cell phones were a lot newer, some people wanted to jam cell phones on their private property, to prevent disturbance of other patrons. The FCC dealt with these cases under provisions of the Communications Act. However, Title III of the Communications Act (302A) governs only devices that actually transmit radio signals. And Title II of the Communications act (214) permits temporary discontinuance of service. Thus, passive actions, such as installing wire mesh in the walls of a restaurant or movie theater, so it acts as a Faraday cage, would appear to be legal. Disconnecting some antenna (nodes) of the cellular network (those that work in the subway) would also appear to be legal, if it is brief enough.
So I don't see many grounds for legal action against BART, but IANAL, although I have played one in the hearing room.