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When the air brakes are released on a car, the pressure in the cylinder that provided the braking effort has to be exhausted to the atmosphere. This air passes through a small valve called a retainer, which has four positions. The normal is called release, which allows the exhaust to pass directly to the atmosphere. The other positions allow some of the pressure to be "retained" in the brake cylinder, which means that the brakes on the car are still on even though the control valve is in release position and the brake reservoirs are being recharged from the brake pipe. These other positions yield varying rates of exhaust of the air from the cylinder, according to the conditions to be dealt with. The retainers have to be set up by hand, and are used on long grades to keep the brakes on the cars - and the train from running away - while the reservoirs are recharged for the next brake application. They're "turned up" (set to retain) before descending the mountain, and are "turned down" at the bottom, for normal operation. They're a great safety device - ask any mountain hogger. Old Timer
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