"A pressure retaining valve, which is connected by piping to the triple valve exhaust, it's purpose being when the handle is placed in retarting position, to retard the rate of brake cylinder exhaust while the engineman is recharging the auxiliary reservoirs, and when the brake cylinder pressure has been reduced to a certian predetermined amount, to retain that pressure in the brake cylinder."
from "Single Capicity Freight Brake Equipment ("K" Triple Valve)"
More commonly refered to a retainer they are usually "set up" before decending a steep grade. The brakes on the cars will remain released until the engineer makes the first application. when the engineer releases the brakes the trainline pressure rises and begins to recharge the auxiliary reservoirs but the brakes remain applied holding the train instead of releasing quickly. The retaining valve contains a spring actuated valve that closes when the cylinder pressure drops to 10psi. The air also hase to pass through a "chock" that restricts the flow and gradually releases the pressure until the spring shuts off the flow.
10 psi is roughly the cylinder pressure of a minimum application. SO when the engineer is ready to apply the brakes again the Auxiliary reservoirs are full and the brakes are still applied with the shoes warmed up and ready to go, or stop, which the train must do when it reaches the bottom of the grade.
They remain applied until a trainman manually releases the retainer on each car at the bottom. This allows the engineer to adjust the braking force as the grade changes with out ever going into full release.
The retaining valve is a part of all modern freight brake systems, and passenger cars that saw steep grades were also equiped.