If a subway train goes dead while it is running between stations, what could the train operator conclude if the main lights were dark?

When a subway train experiences a total loss of power while running between stations and the main lights are dark, this situation suggests a failure in the power supply system. By observing that the main lights are off, the operator can conclude that the train is not receiving electrical power, which typically comes from the third rail.

If the third rail were functioning properly, the train would still have power, at least sufficient to keep some electrical systems operational, including the lights. Therefore, if the lights are completely dark, it indicates a significant issue with power delivery, supporting the conclusion that the third rail is dead or unable to supply power to the train. This reasoning helps eliminate other possibilities, as low battery power wouldn’t lead to lights being out entirely, and refueling is not relevant to electric trains. A malfunction in the train's system could potentially cause issues as well, but the immediate and evident symptom of darkness points directly to the power supply from the third rail being at fault.