Tinplate Art posted:Another interesting term is "peddling off the air". Maybe the experienced rails here could explain that potentially dangerous procedure here as well?
On every car the air used to pressurize the brake cylinder, which applies the brakes, comes from the auxiliary reservoir. On a freight train there is only one pipe that runs the length of the train. The pressure in that pipe is reduced to set the brakes and increased to release the brakes. The auxiliary reservoir is recharged when the brake pipe is at the release pressure. The recharging of the auxiliary reservoir is done through a very small choke so the air used for recharging does not effect the operation of the brakes. On a long train there could be 100 or more auxiliary reservoirs recharging at the same time, so each reservoir gets only a tiny amount of air.
When going down a long grade the engineer cannot repeated apply and release the brakes as that would slowly reduce the auxiliary reservoir pressure on every car as they are not having enough time to recharge to the full pressure. This reduces the amount of braking available and can result in the train running away due to a lack of braking. This may be referred to as the term you referred to.
The engineer has really three choices if dynamic brakes are not available. One is to make long significant brake applications on steep parts of a hill and long recharging times on less steep areas. Second choice is to stop the train and hold it with engine brakes and hand brakes while recharging the auxiliary reservoirs. The third option is to set retainers, which restrict or plug the exhaust from the brake cylinders keeping the brakes applied, while the brake pipe is in release and the auxiliary reservoirs are being recharged. The retaining valves have to be manually set on each car at the top of the hill and manually released at the bottom of the hill. All three of these options could make for a very slow trip.
