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August 1997 - Page 48


Brian Solomon

The work of the air brakes is revealed in the smoke pouring from the wheels of this eastbound Conrail train, inching downgrade through the Berkshire mountains of western Massachusetts.

ere is a (very simplified) description of how North American freight train brakes work and some an swers to other frequently asked questions. Basically, there is a reservoir ( air tank) on each car w h i c h is c h arged w i t h ( no m i n a l l y ) 90 psi o f compressed air, supplied b y compressors o n t h e lo comotive and sent to every car through the train's b r a k e l i n e . ( Th a t 's the h o s e you see l i n ki n g t h e cars-the one t h a t goes "kapoosh ! " when t h e train uncouples . ) Once the reservoirs on all the cars are charged, the engineer can set the brakes on the en tire train by bleeding air out of the brake pipe , using a valve i n the locomotive cab. The reduction of air pressure i n the brake pipe causes a valve on each car

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to connect that car's reservoir air to the brake cylin der on that car, applying the bra kes. To release t h e brakes, the engineer moves the valve t o t h e Release p o s i t i o n , w h i c h once aga i n sends compressed air b a c k through the tra i n . The i ncrease in pressure in the brake p i p e causes the valve on each car to vent t he air in the brake cylin der to the atmosphere. A spring in the brake cylin der of each car c a u s es the brakes to move away from the wheels . T h e brakes apply whenever t h e air pressure in the brake pipe drops. If the train accidentally uncouples, the brakes wi l l automatically apply fully-since all of the brake pipe pressure will be vented to the at-

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