Final answer:
When third rail power is interrupted, the immediate effect is that the train loses its power source and will come to a stop.
Step-by-step explanation:
When third rail power fails, one IMMEDIATE effect on a train is that the train will lose its source of power and come to a halt. Third rail systems provide electric power to trains using a conductor rail placed alongside the tracks. Without this electrical supply, the train's traction systems cannot operate. This is similar to how a circuit works; if the supply of electricity to a circuit is cut, all the charges stop moving, much like how a train stops when the third rail power is interrupted.