Noun
retaining valve (plural retaining valves)
(rail transport, mechanical engineering) A device that retains some air pressure in a train's air brake cylinders even when the automatic brake valve is set to the fully-released position, used on steep downgrades to allow a degree of continuous braking effort to be maintained without depleting the pressure in the train's brakepipe (which would result in the loss of nearly all braking ability, causing a runaway).