So you have the two rails the train travels on, and then a third "rail" which is really just a thick (like, several inches thick) wire carrying about a thousand amps of current. The train has another wire that drags along this third rail as it moves, drawing current and making the motor moves.

Since it takes about 1 amp of current to kill a person, touching the third rail is usually a pretty quick way to get yourself killed.

They definitely aren't!

A few years ago I was driving up to Boston on July 4th and there was no one else on the road except some jackass in a Nissan SUV with NY plates who would accelerate to 90 whenever I tried to pass and then SLAM on the brakes every time he saw a toll transponder. At least he was predictable, as far as brake checks go.