So we're always told not to touch the third rail, as it's at a very high potential. Makes sense. But how does return current work in such a system? Let's say there is some load (the motor - we'll simplify and assume DC). One terminal of the motor is connected to the third rail, what's the other end connected to? My friend theorizes ground... but that seems wrong to me. You can't just charge up the earth through the motor... can you? Could that work? Why not? It seems to me one of the rails ridden upon must be used for return current - and this would seem to imply that if you were between the train and the generator and you touched said rail, you'd be electrocuted.