Why must hand brakes be applied when air brakes may leak off?

Hand brakes must be applied when air brakes may leak off primarily to compensate for potential failures in the air brake system. If the air brake system experiences a leak, it can lose pressure, which may result in reduced braking capability or complete failure to stop the train effectively.

By applying the hand brakes, you create a physical mechanical force that acts as a safeguard, ensuring that the train remains stationary until it is ready to move. This is crucial for safety, as it prevents unintended movements that could occur due to the loss of pressure in the air brake system.

While the condition of the air brake system can impact other aspects of train operation, such as overall stopping power and safety during station stops or when parked on an incline, the hand brakes provide a reliable secondary method of controlling the train's movement in the event of air system failure.