Freight Car Braking Systems

Hello all...

I'm trying to understand the science behind the air braking system on modern rolling stock, and I now have a good understanding of how everything works. Pressure in the train line releases brakes in the rolling stock, drop in air pressure causes the reservoir to force air into the brake cylinder, applying brakes. That's all well and good, but I have a few lingering questions:

1. When cars are spotted at a customer or on a siding, either in cuts or individually, I assume that means all of the air has bled out of that car's brakeline. Do the brake shoes continue to compress against the wheels until the airline is charged again? Or do the shoes release once the air reservoir runs out of air and the brakes gradually let loose?

2. What's the purpose of the brakewheel, then, if a car's brakes are already activated when it is not part of a pressurized consist? I recall reading of a few accidents where cars have rolled because the brakewheel had not been set. Shouldn't that car's brakes have been on if there were no pressure in the brakeline?

3. What about hump yards and letting cars roll... as soon as the car uncouples from a loco or another car, the trainline is severed and technically that car's brakes should automatically and immediately be activated. If so, why does it still roll?

Any details you can provide on the setting of a freight car's brake system when the car is on its own (not part of a train) would be greatly appreciated.

2. The brakewheel is like a automobile's parking brake. When a set of cars are left on a siding the handbrake (aka brakewheel) is set. By setting the handbrake, you tighten a chain attached to the brake lever. The brake lever is attached to the brake shoes. By tightening the chain, you pull the shoes to the wheel. The handbrake has a lock that prevent the chain from loosening until a switchman manual removes it.

3. Each end of the car along the brake line has a lever that allows the switchman to close the air off. This "traps" the air in the car, allowing it to freeroll in a hump yard.

I'm trying to understand the science behind the air braking system on modern rolling stock, and I now have a good understanding of how everything works. Pressure in the train line releases brakes in the rolling stock, drop in air pressure causes the reservoir to force air into the brake cylinder, applying brakes. That's all well and good, but I have a few lingering questions:

1. When cars are spotted at a customer or on a siding, either in cuts or individually, I assume that means all of the air has bled out of that car's brakeline. Do the brake shoes continue to compress against the wheels until the airline is charged again? Or do the shoes release once the air reservoir runs out of air and the brakes gradually let loose?

One important step not mentioned above is that when the brake pipe is charged, the brake reservoir on each car is replenished with compressed air. This can only be done when the brakes are released. So yes, unless the system is recharged from the brake pipe, the brakes on an uncoupled car will leak off and the brakes will release. Depending on the leak rate in an individual car, this can be a few hours to a day or so. That is why hand brakes are always set when cars are left by themselves. It is not unheard of for one or more cars to run away, sometimes going for miles, when the cars are left on a main track without setting the hand brakes. After a while, the brake leak off and the cars start to roll downhill.

For a great explanation of freight car brakes follow this linkhttp://www.railway-technical.com/brake2.shtml
any body with an interest in how freihgt brakes work will appreciate this.

3. Each end of the car along the brake line has a lever that allows the switchman to close the air off. This "traps" the air in the car, allowing it to freeroll in a hump yard.

You will find such humping is usually done with the cars bled off - air drained from the cylinders and no air in the brake pipe. While the procedure you describe will work in theory, a surprising number of cars have slight (or not-so-slight) leaks in the angle cock, brake pipe, or other location. If you tried humping a long cut of cars using "bottled air", you would spend half your time dealing with cars which got halfway down the hump and had the brakes set up from leaks.

BR&P wrote:

3. Each end of the car along the brake line has a lever that allows the switchman to close the air off. This "traps" the air in the car, allowing it to freeroll in a hump yard.

You will find such humping is usually done with the cars bled off - air drained from the cylinders and no air in the brake pipe. While the procedure you describe will work in theory, a surprising number of cars have slight (or not-so-slight) leaks in the angle cock, brake pipe, or other location. If you tried humping a long cut of cars using "bottled air", you would spend half your time dealing with cars which got halfway down the hump and had the brakes set up from leaks.

Ohhhhhh..... Had not thought of that. Makes sence though. So, you bleed the air off and do not set the handbrake, then push the cars over the hump and let them free fall.... Or is there more to it?