How much should the rear brake pipe pressure decrease to indicate that brakes are applied?

To indicate that brakes are applied, a decrease of at least 5 psi in the rear brake pipe pressure is necessary. This is significant because it ensures that the braking system has effectively engaged, allowing for the necessary force to be applied to the train's wheels. A decrease of this magnitude is typically sufficient to confirm that an action, such as applying the brakes, has taken place within the system.

Lower decreases may not reliably indicate that brakes are fully engaged, as smaller fluctuations can occur due to various factors such as system pressure variations or gauge inaccuracies. Therefore, the threshold of 5 psi serves as a reliable standard to ensure that the braking command has been fully transmitted and that the brakes are indeed activated, promoting safe and effective train handling operations.

This understanding is essential for train crews, as it helps them accurately assess brake functionality and respond appropriately to ensure safety.