What device on a freight car helps retain brake cylinder pressure for retardation?

The retaining valve is essential for maintaining brake cylinder pressure on a freight car, allowing for effective retardation during operations. When engaged, it prevents the quick release of brake pressure, which is particularly beneficial when descending grades or when additional retarding force is needed. This device ensures that the brakes remain effective longer and enables train crews to manage speed control more efficiently when descending hills or during other situations where additional braking is necessary.

In contrast, the pressure relief valve functions to prevent excess pressure buildup by releasing pressure when it exceeds a certain threshold, acting as a safety mechanism rather than aiding in brake retention. The brake cylinder accumulator serves a different function by providing a backup or additional pressure source for the braking system but does not retain pressure on its own. The service brake valve is responsible for controlling the release and application of brakes within the standard operating procedure of the braking system but does not focus specifically on retaining pressure for retardation purposes.