Tim+ expressed precisely :
Today we rode a train with two overhead wires but no resistor banks. This
train apparently runs on a three phase supply. Presumably the rails supplied
the third "leg" of the supply but how safe is this with AC? Wouldn't the
rails be dangerously live relative to earth?

It could have been single phase and neutral on the wires, or two phases
and an earth via the earthed rail.

The motor would likely be synchronous, driving a worm gear. The drive
would only allow the car to decend limited by the motor's speed.