By mass transit I assume you mean the likes of the New York City subway, not intercity High Speed Rail or commuter rail.
In that case the distances are too short for the trains to get to any sort of high speed without pulling eye-popping 'G's under acceleration and brakes. Even commuter rail doesn't really benefit from high speed, when they sped up the commuter rail here in Victoria only 32km of the 80km Melbourne-Geelong route is 160km/h, the rest is slower through outer suburbs sharing lines with the Melbourne Metro trains.