Final answer:
In case of a power failure from the third rail, the train operator should follow emergency procedures, engage backup systems if available, communicate with the control center, and ensure passenger safety, possibly using an emergency brake and evacuation if needed.
Step-by-step explanation:
If the power goes off the third rail while a train in regular passenger service is moving between stations, the train operator should first remain calm and follow the established safety procedures. The train's momentum will carry it forward for a time, but the operator should engage any onboard backup systems, if available, to safely control the train's movement. They should communicate with the control center to inform them of the situation and get instructions. The operator might need to use the train's emergency brake to bring it to a controlled stop if it's necessary to prevent an accident. The safety of the passengers is the top priority, and they should be informed about the situation in an orderly manner to keep calm. Once the train has stopped, the operator should continue to follow emergency protocols, which may include evacuation procedures if the power cannot be promptly restored.