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August 1997 - Page 54


Brian Solomon

ABOVE: The prominant position of the air brake pressure gauges is shown in this view of the control stand on an F45, as well as the position of the brake valve relative to the throttle and dynamic brake handle. LEFT: When braking is no longer needed, the engineer moves the valve handle back to Release & Charge. This starts to increase the pressure in the brake pipe, releasing the brake cylinder pressure to the atmosphere via the retainer valve. The retainer valve can be manually set to hold some pressure in the cylinder until manually released. To help release the brakes quickly, pressure from the emergency reservoir is vented into the brake pipe. RIGHT: With the pressure gone from the brake cylinder, the compressor continues pumping the air into the brake pipe and reservoirs in order to restore the system to 90 pSi. BELOW RIGHT: Brake components are easily seen on a tank car where they cling to the framing. Visible on this car is the control valve (on the len side), the brake pipe running the length of the car towards the bottom, underneath the car are the rods connecting the brake cylinder to the brake rigging on the trucks.
t he head end releases first and moves away from the s t i l l-anchored rear portion . I n t h e early days, t h i s problem was solved by p u t t i ng c ho kes in t h e pipes carrying a i r from t h e brake pipe to the reservoir o n each car. This allowed a more rapid build-up of air pressure in the brake pipe all the way to the rear of the tra i n , since each car reservoir was consuming brake pipe air at a slower rate because of the choke restriction as it recharged following a release. But as trains got longer and heav ier, this was not enough, and the chokes slowed down the initial charging of the trains in the yard and the recharge on the road. Here comes smarty again, and l i ke a Congressman eyeing . the Social Security trust fund , he can't stand to see a surplus go unused. Remember that emergency reservoir on each car? I t was initially charged t o 90 psi and never used if the en gineer did not need an emergency application. Hmm mm. All that air there . . . . The triple valve was modi fied again so that when a car goes into release, it vents the cylinder air to atmosphere, releasing the brake as before; connects the brake pipe to the reservoir to begin re-charging, as before; and connects the 90 psi emer gency reservoir to the brake pipe to boost the pipe pres sure quickly at each car. This results in fast releases throughout the train, but it depletes part of the emer gency air if it is needed before the system can recharge. And, basically, that's how train brakes work today. Remember the leaky reservoir question? With an entire train of this type of equipment, what are the an swers to the two questions posed in quiz No. 1 under the same conditions? Answer: same as before-the brake will release on the leaky car. However, that one leaky car will dump its emergency reservoir air into the brake pipe when it moves to the release position, rais ing the brake pipe pressure on that car a nd on the cars next to it! W hen the cars nearest the leaky one see the brake pipe rise slightly above their service reservoir pressure, their valves interpret this as a release signal and they also move to release! Now they also dump their emergency reservoir air into their brake pipes , triggering the adjacent cars to release as wel l. Because of that single leaky service reservoir, the entire train will release. It is for this reason that it is against the rules to "bottle the air" (close the brake line angle cock on the train) when uncoupling the engines.

O n long steep grades it may be necessary to set and release the brakes several t i m e s because of grade changes, etc. But i f the brakes are released on a steep hill, the train immediately accelerates. If the engineer

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