During a brake test, what indicates that the brakes on the rear car have released?

The observation that the brake pipe pressure at the rear car increases by at least 5 psi is an effective indicator that the brakes have released. In a typical air brake system, when the brakes are applied, the brake pipe pressure drops. Conversely, when the brakes are released, the pressure in the brake pipe will begin to rise as air is allowed to flow back into the system. An increase of at least 5 psi signifies that the air brakes are no longer engaged and have fully released, thus allowing for proper functionality during train movement.

This understanding of pressure dynamics is crucial for ensuring the reliability of the brake system and for maintaining safe train operations. It helps conductors and engineers confirm that the brakes are functioning as intended before the train departs. The other indicators, while they may suggest a change in the brake status, do not provide the same level of precise information about the release of the brakes as the increase in brake pipe pressure does.