What is the primary function of the retaining valve in a freight car air brake system, and how does its failure affect braking performance on a descending grade?

The primary function of the retaining valve in a freight car air brake system is to maintain a residual air pressure in the brake cylinder after a brake release, providing a supplemental braking force, especially on descending grades. A retaining valve does this by restricting the exhaust of air from the brake cylinder when the train's air brake system is released. This maintained pressure helps control train speed and prevents excessive wheel heating from repeated full service brake applications on long downgrades. When a retaining valve fails, it....