I have been mislead. I always thought that train brakes automatically engage if the air cuts off. Or to put it another way, I thought that the compressed air kept the brakes disengaged, then when the air was released the brakes would apply.
Sort of. There is a air tank on each car that needs to be filled with compressed air. When the pressure in the train line drops, the brakes come on. The issue arises in that the system isn’t completely air tight. So if I disconnect a car, with a full air tank, as soon as the pressure in the train line is released the car’s brakes come on. But if I leave that car sitting there, with just the air brakes holding it, then eventually the little leaks in the system deplete the tank, and the brakes release. So that car will roll away if its on a grade. Not right away, but eventually. Minutes or hours later that car will be gone
David Maynard said:
Sort of. There is a air tank on each car that needs to be filled with compressed air. When the pressure in the train line drops, the brakes come on. The issue arises in that the system isn’t completely air tight. So if I disconnect a car, with a full air tank, as soon as the pressure in the train line is released the car’s brakes come on. But if I leave that car sitting there, with just the air brakes holding it, then eventually the little leaks in the system deplete the tank, and the brakes release. So that car will roll away if its on a grade. Not right away, but eventually. Minutes or hours later that car will be gone
Dave, I’m not sure that I read your reply correctly. Your saying that the air pressure keeps the brakes off. When the air pressure is cut off, the brakes apply. But then you said that if a parked car’s air tank bleeds, the brakes will disengage.
Dan, almost. When the car is hooked up to a locomotive the car’s tank is empty, so the air brakes are not applied. Then the locomotive pumps up the tank through the train line, pressurizing it. When the pressure in the tank and the pressure in the train line is the same, the air brakes are off. This is controlled by the valve, be it a K valve or AB valve, depending on the vintage of the car. Now when the pressure in the train line is reduced, the valve releases some air from the tank to the brakes, applying the brakes. The more the reduction in the train line, the more the application of the brakes. But the muscle to apply the brakes comes from the air tank on the car. So as the air slowly leaks out of the tank, there is less muscle to apply the brakes.
By using a reduction in train line pressure to apply the brakes, the brakes have a fail safe. If the train seperates for any reason, the brakes are applied.The orginal design was to use pressure to apply the brakes, but any brake in the train line would render the brakes inoperative. So George Westinghouse redesigned his air brakes with tanks on each car, so that a reduction in air pressure would apply the brakes.
Its the leakage in the system that causes issues. If a train is going down a long grade and the train line is say at 90 pounds of pressure, and the engineer reduces the pressure in the train line by 15 pounds (applies the brakes) then each car puts that 15 pounds of air pressure int their brake chambers, using air from their tanks. But as some of it leaks off, some cars start braking with less force. So lets say a car leaks off 15 psi from its tank, now its tank and the train line have the same pressure, so no brakes are applied on that car. Now the engineer has to reduce the pressure in the train line some more to get that car to apply its breaks. On a long downgrade, with a leaky car, this can cause issues.