Air valves on freight cars that do not allow the air brakes to fully release. The retainers are manually set at the top of a hill by the brakemen. When the engineer applies the brakes, if he releases them, some of the brake pressure is “retained” and the brakes remain applied on the car. This allows the brake system to be recharged without fully releasing the braking action. At the bottom of the hill, the retainers have to be released manually by the brakemen and the wheels on the cars cooled. Because the train has traveled down the hill with the brakes applied, the brakes and wheels are very hot. Retainers were more commonly used during the steam era, the advent of dynamic brakes made them less necessary. In the modern era they are only used on the steepest of grades.