Page 18 - June 2020 Track N Times

TECHNICAL TRAINING

Three Position Retaining Valve, by Ralph Spicer, Manager Operational Standards

When the air brakes are release on a car, the pressure in the cylinder that provided the braking effort must be
exhausted to the atmosphere. The air passes through a small valve called a retainer, which has three 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(A9 or trainline) 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 a 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 devise – ask any mountain engineer (hogger).

EX - Exhaust
SD – Slow Direct

HP – High Pressure Retain

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