When must hand brakes not be released after an engine is re-coupled?

The correct choice emphasizes the necessity of ensuring that the air brake system is fully operational before releasing the hand brakes after an engine is re-coupled.

In the rail industry, the air brake system is critical for controlling and stopping the train safely. If the hand brakes were to be released before the air brake system is charged and functional, there is a significant risk that the train could not be adequately stopped or controlled. The air brake system needs to build pressure to provide reliable braking force, so it's essential to confirm that it is properly charged before proceeding. This safety procedure helps prevent accidents and ensures that all operations are conducted smoothly and securely.

In contrast, the other options point to important processes, but they do not specifically address the core concern of safe braking and control. While checking for secure attachment of cars, confirming speed, and waiting for the conductor's signal are all critical parts of operating trains safely, ensuring the air brake system is active and functional takes precedence in the context of releasing hand brakes after re-coupling.