On a recent trip via public transit in the US, I found myself in a subway car so jam-packed that there was scarcely room to breathe. The car was jerking about quite a bit, and there was little to grab on to other than your fellow passengers. This is a fairly regular occurrence on this line, and they do not seem to enforce any capacity limit other than "everyone must be inside the doors." This strikes me as extremely unsafe. If the train had crashed, or there had been a fire, there is no way we would have been able to get out of there. I have to believe this has to be against some sort of safety regulation, but I have so far not been able to find anything applicable. Are there really no laws governing train car capacity?
I don't believe there is any federal law that covers this. Even if there were, BART says that it can carry "over 200 customers in a crush load." So that's comforting.
A2 cars can only operate as lead or trail cars in a train consist. A2 cars have an operator's cab, automatic train operating equipment, and two-way communications system. The A2 car seats 60 customers comfortably and can carry over 200 customers in a crush load.
B2 cars can only operate in the middle of a train consist; they do not have a cab nor do they control the operation of the train. They can, however, carry the same customer load as an A2 car.
C1 cars can operate as either lead, trail or in the middle of a train consist. They are equipped with an operator's compartment, automatic train control equipment and a communications system. C1 cars give BART flexibility to change train size without rerouting to a storage yard. Seating capacity is 56, but can carry over 200 customers in a crush load.
C2 cars are nearly identical to C1 cars. Seating capacity is 56, but can carry over 200 customers in a crush load.
Regulation of public safety is generally reserved to the states, with the federal government having an interest only if a state line is crossed or if it involves a federal property.
You mention it was BART in San Francisco, so the primary regulator is California.
Not being expert on California, I'm not sure how much power has been delegated to county or city governments, but federal involvement is unlikely.