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August 1997 - Page 50


RELEASE & CHARGE

LEFT: On the traditional 10comotive control stand, the engineer's brake valve is located alongside the engineer's seat. This han dle (on a GP30) is painted the appropriate color, red.
On these and the following pages, a typical sequence of air brake functions is illustrated using the dia gram detailed on the pre vious page. To show each part's role in each action, air pressures are color coded; red denotes rising air pressure, green indi cates steady or constant air pressure, and blue shows falling air pres sure. For clarity, not all parts are shown. FAR LEFT: Once a train is assembled and before it departs, the locomotive "pumps the air" until the brake pipe and all reser voirs are charged to the amount of pressure speci fied by the engineer ac cording to the feed valve. mGHT: With fUll pressure established (in this case 90 psi), the control valve moves to Lap position and once tested with a trial brake application, the train is ready to depart.
Dean Sauvola

Train Control
T he engineer can "graduate" the brakes on-that is, control the braking force in the "more brake" direc tion, but any release can only be a full release. He or she can't graduate a freight train's release as a car driv er does by easing up on the brake pedal-one of the many characteristics that make running a train differ ent from driving a car or truck. The engineer must not set too much brake, or he will stop short. Too little brake and he can set some more-if there is time. I f there isn't time, h e i s fired-or dead! P a s senger cars are a l i t t l e d i ffere n t : some a re equi pped w i t h graduated release brakes , and some even have anti-skid brakes. However, you won't find either feature on a freight car. ( Some freight cars do have a load/empty sensor. Brake cylinder pressures can be higher when loaded and limited when empty to help prevent wheel sliding during braking.) Locomotives have two air brake systems-one oper ates in concert with the train brakes, and the other as an independent brake exclusively for the locomotives.

The independent brake uses additional air brake hoses connected between units, is faster-acting, and offers graduate-on graduate-off control. This locomotive-only brake is usually used only during switching, light engine moves, or for holding a stopped train on level hack. Most locomotives have a third braking system-dy namic brakes-which essentially rewire the h'action mo tors to act as generators. The electricity generated is thrown away as heat in locomotive resistance grids. Generating this power creates rolling resistance, slow ing the rolling train. Trailing locomotives are controlled by the engineer in the lead locomotive's cab and automatically do what ever the lead locomotive is doing, controlled via a multi wire jumper cable connected between each locomotive. Ah, but then wouldn't a small locomotives be pulled or pushed by larger locomotives in the same consist? As it tUtTIS out, multiple unit locomotives of differing horsepower operate perfectly well together. Each will do its share of the work. Suppose a man is pulling on a rope tied to a heavy wagon. The wagon is too heavy for the man to move alone. H i s 1 2-year-old son comes along and begins pulling on the rope, and the wagon

50. August 1 9 97

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