>>1429947An electric circuit needs a live line and a return line. Hence a circuit. Think of it like a pump, water pipes and a water wheel,where you want to pump water through the pipes to move the water wheel. The pump needs the water to return after moving the water wheel so that it can pump it again, so a circuit of water. Roughly, that's how the electrical circuit works, it needs the electricity to return to the generator. Railways use the rails for this return, but trolleybuses can't, so they use the second wire. Does that clear it up?
Now a streetcar can share the live wire of a trolleybus overhead line, as in the pic, and it still uses the rails for the return, while the trolleybus uses the second wire.
A problem would arise if you somehow connected the two wires directly, as the circuit would overload as you've removed the resistance. Like if you remove the water wheel from the water circuit and the pump overloads because the water moves ever faster, that would be the simile. Hence: short circuit. The circuit lacks a resistance and overloads. Thats where the circuit breaker switch comes into play, it breaks the circuit to prevent the electrical system to catch fire from the overloading heat.
The pantograph is simply a contact surface, but it's wide, so if it were running on the trolleybus wire it could touch the return wire, thus connecting both and causing a short circuit.
