retaining valve

Engineering, Mechanical, Mechanical engineering, Natural sciences, Physical sciences, Rail transport, Railways, Transport 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).