What method cannot be used to determine if the third rail is dead after coasting for some distance?

The method that cannot be used to determine if the third rail is dead after coasting for some distance is blowing the train whistle. This action does not provide any information about the electrical status of the third rail. The whistle is primarily used for signaling purposes and has no connection to the train's electrical systems or the functioning of the power supply from the third rail.

In contrast, other methods mentioned, such as using the circuit breaker, checking lights on the control panel, and observing train response, are all relevant to evaluating the power supply. For instance, the circuit breaker can indicate whether there is power flowing through the system. The lights on the control panel can reveal the operational status of the electrical systems, and observing the train's response can help assess if the traction power is operational or if the train is coasting due to a loss of power.