Re: Air Brakes

If I may, I'll try to help you out here. First of all, both straight air and automatic air systems accomplish the same thing, namely provide the force to press the brake shoes on the cars against the wheels. But they accomplish this in somewhat opposite ways.

The straight air systems were the first to appear, and were relatively simple. The air pressure was supplied and controlled from the engine, and from a reservoir on it. These systems disappeared fairly quickly because of a basic flaw. Any break in the continuity of the air line would disable the brakes on the entire train.

Automatic air systems overcame this flaw. The brakes are set not by an increase in the train line pressure, but a reduction. This sounds contradictory until you closely examine the makeup of the automatic air system. On each car is a controlling - triple - valve which automatically charges up a small reservoir. As long as the train line is intact everything is balanced until the engineer sets the brakes by reducing the pressure in the train line, allowing air from the car reservoirs to be channeled to the piston which provides the mechanical pressure to set the brake shoes against the wheels.

The brakes are released when the train line air pressure is higher than the car reservoir pressure. The big, big, advantage of this system is safety. If for whatever reason - like a broken air hose - the train line pressure goes to zero, the air in the car reservoirs - through the triple valve - automatically set the brakes beyond the break to emergency and thereby stopping the train. This principle has gone pretty much unchanged to this day.

A modern day variation is the so called electro-pneumatic systems, found principally on passenger equipment. Here, the operation of the car triple valves is controlled by an electrical signal from the locomotive. The big advantage here is that the brake response is essentially instantaneous throughout the train, rather than responding to a "pulse" or change in the train line pressure. In a standard system the brakes are set starting at the head end and progressing down the train as the "pulse" reaches each car. In a long train it may take several seconds for the "pulse" to travel the length of the train, which means that the brakes on the head end may be set while on the rear of the train are still released until the "pulse" reaches them. This of course causes the slack in the train to run in and out, and rough handling can cause broken couplers.

Hope this helps.

CJ ("Chile" John. Notice the spelling. )