In which position does the retaining valve exhaust all brake cylinder pressure?

The retaining valve exhausts all brake cylinder pressure in the Direct Exhaust position. This position is specifically designed to allow for the rapid release of brake cylinder pressure when needed. In this mode, the retaining valve bypasses any restrictions that might hold back the release of air from the brake cylinder, enabling a quick drop in pressure that can be essential during certain operational scenarios, such as when transitioning from brake application to release when trains are descending a grade or when quick stopping is required.

In contrast, the other positions, such as the Standard position, High Pressure position, and Controlled Release position, serve different functions. The Standard position maintains a balance of pressure, the High Pressure position keeps the brake cylinder at a higher level for more aggressive braking, and the Controlled Release position allows for a slower and more gradual release of pressure. Each of these has its purpose in managing braking effectiveness and response but does not fully exhaust the brake cylinder pressure as the Direct Exhaust position does.