When must the cab signal apparatus be tested?

The cab signal apparatus must be tested within 24 hours before the engine leaves its initial terminal to ensure the system is functioning correctly and will provide the necessary updates to the crew during operation. This timing is critical as it ensures that any malfunctions can be identified and addressed before the train departs, thereby enhancing safety for both train operations and personnel. Regular testing within this timeframe is part of operational protocols to maintain a high standard of safety and reliability, making it a fundamental requirement in the rail industry.

Testing the cab signal apparatus ensures that signals are accurately transmitted and received, which is essential for safe train movement, especially in areas where cab signals control speed and stop commands. This preventive measure helps to avoid potential accidents due to signal failures while the train is in transit.

The other options do not reflect the safety protocols established for cab signal testing, as they do not ensure the necessary level of readiness and safety within the mandated timeframe.