Power reverses always run on air. Some have an emergency steam line, tapped into the turret and running down to the power reverse, to use in case of an air system failure. This allows the engineer to throw the engine into reverse to get stopped if he is running lite, and there is a failure in the main reservoir piping. The cups in the power reverse are not meant to work with steam, and this valve is to be opened only in a real emergency. The lack of an emergency backup is what did in the D&SL 210 on the West Side of Rollins pass. While drifting down the hill, the engine threw a rod, which punctured the main reservoir, draining it instantly and leaving them with no brakes, and no power reverse. The crew unloaded immediately, and the engine ended up far down the mountainside, never to run again.
For normal operation, the engineer works a miniature version (about 12 or 15 inches tall) of the Johnson bar that all of the other engines have. This operates the control valve, which is mounted right on top of the power reverse. This control valve is what holds the valve gear in the desired position by pressurizing both ends of the power reverse cylinder. How precise the control is depends on the setup of this linkage, how freely the control valve moves, and the condition of the piston cups in the power reverse.
If the reverse lever is mounted rigidly to the boiler, if the reach rod to the power reverse has a straight shot, if there isn’t too much lost motion in all of the linkage, if the control valve isn’t too stiff, and if the cups don’t leak, it can be very precise. If not, it’s not. I’ve seen reverse levers that flex on their boiler mounts, reach rods that resemble a pretzel to get around obstructions, and very stiff control valves that conspire to keep the valve gear hunting all day long.
When you compare all that to simply dropping the Johnson bar into the right notch, directly locking the valve gear into position, it’s easy to see that if the engine is small enough to be able to run without a power reverse, you better do so.
Our summer time railroading sometimes keeps us from seeing all the reasons that the railroads did things. I have run an engine with a Johnson bar that could be set by one hand with relative ease into the desired notch, on a nice sunny day. I have also done switching with that same engine on a below zero day, with the wind blowing, where each change of direction required both the engineer, and the fireman pulling for all that they were worth. We would go out and oil the links about every five minutes, which would free it up for about one minute. If we would have had to put in a 12-hour shift doing that, every day, no doubt there would have been screaming for a power reverse in short order.
