>With a retaining valve, the brake cylinder pressure is held, on that car, while the brake pipe is recharged. In effect it holds some of the cars set up, while the others release and recharge. Rule 17 on the SP stated, "If at anytime in Engineer's judgement use of retaining valves is required, stop will be made and retaining valves will turned up in accordance with his request."

To expand a little on this: a retaining valve essentially plugs a car's brake cylinder exhaust so the brakes remain applied while its auxiliary reservoir is being recharged, and this occurs only on cars which have had their retainers turned up. That statement can be accused of gross oversimplification because depending on the era and design of retainer, they held the pressure, released it slowly, or could be set for more than one pressure to be retained (in the brake cylinder).

When the RRs had passenger service, all the passenger cars had retainers; Amtrak cars don't have them.