Running the locomotive in reverse motion with the drifting throttle valve open admitting steam into the cylinders is the standard way to bring an engine down long grades where continuous use of the locomotive brakes would be necessary to keep control. Using the airbrakes on a locomotive continually can heat the tires and cause them to come loose on the centers.
Simply putting the locomotive in reverse motion while moving causes the pistons to suck hot smokebox gases and cinders into the valves and cylinders. Then the pistons would compress this gas to a higher temperature, destroying the lubrication in the valves and cylinders, chewing up piston and valve rings.
Early on the locomotives had "water brakes" which used wet, low pressure steam, piped off the boiler (actually the tap for the steam was below the water level - the hot water flashed into steam in the pipe) and into the exhaust passages in the saddle. With the locomotive is reverse motion, the pistons would draw this wet steam into the cylinders, instead of the hot gasses, then compress the steam on the reverse stroke, creating back pressure to slow the locomotive. This was a great plan until someone got carried away and opened the valve too far and instead of wet steam he got wet water, tried to compress the water in the cylinders and broke cylinder heads and pistons, bent piston rods, etc.
The Grande came up with the Drifting Throttle, which puts saturated steam from the turret into the steam chests on the admission side of things. The engine is put in reverse motion and the steam is compressed in the cylinders creating back pressure (and heat) to slow the engine, when the piston reverses direction and the valve opens to the exhaust, there is a quick puff of high pressure steam that is sent down the exhaust and up the stack to (in theory) prevent the withdrawing piston from sucking smokebox gasses into the cylinders. One of the things you have to keep in mind is the mechanical lubricators that provide lubrication to the valves and cylinders cannot over come the high pressure of the compressed steam. So, at regular intervals you have to find a nice level spot, or a place with a bunch of curves to help hold you back. The drifting throttle is closed, the air brakes are set, and the engine is put back in forward motion for a while so that the valves and cylinders can get some oil.
Edited 1 time(s). Last edit at 12/10/2016 02:33PM by Earl.
