When should a third rail emergency number not be removed?

The correct response is grounded in safety protocols related to train operations. The third rail emergency number should not be removed until the master controller is off because this action ensures that all electrical power to the train, which is supplied through the third rail, is completely deactivated. Keeping the emergency number in place while the master controller remains active prevents the potential for inadvertent power reinstatement, which could lead to dangerous situations for personnel working on or around the train.

Removing the emergency number prematurely could expose maintenance staff or emergency responders to live electrical components, thus increasing the risk of electrical shock or other hazards. Therefore, the primary focus on safety mandates that the master controller must be off before any action is taken to remove the emergency number.