When does a train operator become aware of a third rail power failure during braking at a station?

The correct choice is that a train operator becomes aware of a third rail power failure during braking at a station as he tries to start after the station stop. This is because once the train has come to a stop, the operator relies on the power supplied through the third rail to initiate movement again. If there is a failure in the third rail power supply, the operator will not be able to successfully start the train when they attempt to move it from a standstill.

Understanding the mechanics of train operation is crucial here; during the braking process and when at a standstill, the operator may not immediately notice an issue since systems can still be functional. However, the moment they attempt to resume travel is when the absence of power becomes apparent. At that point, they would recognize that the train is unable to draw the necessary energy to operate, indicating a potential power failure.

The other scenarios presented do not provide the necessary context for recognizing a third rail power failure specifically at the point of resuming operation, emphasizing why the moment of trying to start after the station stop is the critical moment for awareness.