Final answer:
If a subway train goes dead while it is running between stations, the train operator could conclude that the third rail was dead if the train loses power and stops moving.
Step-by-step explanation:
If a subway train goes dead while it is running between stations, the train operator could conclude that the third rail was dead if the train loses power and stops moving. The third rail supplies electricity to power the train, so if it is dead, the train will not be able to continue running. The train operator can check for other signs of a dead third rail, such as other trains also losing power or passengers reporting a loss of electricity.