A question about brake systems

[basementdweller]

This is something that has puzzled me, I am familiar with air brake systems on highway trucks, air pressure is required to keep the brakes in the released position, if the system fails and air pressure is lost the the brakes activate stopping the truck. If an air brake trailer is parked then it has no air pressure and therefore cannot move. Very simple.
Now I would expect trains to be the same, but if that was the case then the cars couldn't be moved without a train providing air pressure, and that I know is not the case. So if the car moves with no air pressure then surely the air is needed to activate the brakes, (or brake wheel on a siding).
So does the locomotive provide air pressure to activate the brakes? then how does the loco stop if air pressure is lost? I am sure it has to be more complicated than this.
Does a yard switcher hook up air hoses when moving cars? or not.
An explaination of this would be appreciated. Martin

 

[Clerk]

You have noticed the big wheels on the end of the cars. The brakeman turns the wheel which is connected by a chain to the brake system This realeses the brakes so the car can roll. When the car gets close to its spot, the brakeman turns the wheel which sets the brakes.

 

[basementdweller]

Are you telling me that the air brake priciple is the same, except that the brakewheel allows for the car to be moved without air pressure?

 

[Fred_M]

Are you telling me that the air brake priciple is the same, except that the brakewheel allows for the car to be moved without air pressure?

Yes on the hand wheel. But they are different than truck brakes too. Each car has an air supply tank that is pressurized by the loco. When the tank is up to pressure it releases the park brake. The train brake is then controlled by lowering this line pressure which allows a valve on each car to use stored pressure from the air tank to apply the service brake. Trucks have two air lines, 1 to apply brakes, another to charge the air tank. Trains only have 1. See a pdf on my site for an overview of brake parts on older cars. The new ones a a bit different in design, but the principle is the same. http://www.trainweb.org/zmtshortline/airbrakes.pdf Hope that helps. I'm no expert either, just relaying what I have learned. Fred

 

[brakie]

Absolutly..You do not need air to move a car.But,you may have to bleed the car's air reservoir in order for the car's brakes to release..Your hand brake will not do that if the brakes has been set by air.You see the handbrake will be in the release position and not the set position..On the side of a boxcar you will see these words"AIR RELEASE VALVE"..All you do is turn the valve and you will hear the air being release from the reservoir.
As a rule a yard crew will not use air to switch cars.It would take to long plus you would need connect all the air hoses to pump up the air.They simply use the engine brakes to switch with.