What action is required if the cab signal apparatus was de-energized after testing?

If the cab signal apparatus was de-energized after testing, the required action is to retest it before entering equipped territory. This is crucial because the cab signal system is designed to provide critical information to the train crew regarding track conditions and signal aspects. By retesting the cab signal apparatus, you ensure that it is functioning correctly and safely before the train operates in an area where the system is necessary for safe train control.

Retesting confirms that any potential issues that may have caused the system to de-energize have been resolved and that the apparatus is once again capable of delivering the necessary signals. This process helps maintain a safe operating environment, preventing accidents and ensuring compliance with operational standards.

In contrast, the other options do not address the imperative of safety and functional verification that retesting provides. Simply assuming that no retest is needed neglects the possibility that the system may not respond properly, while reporting to a maintenance crew or waiting for dispatcher approval does not directly address the immediate need to confirm that the apparatus is operational before entering the equipped territory.