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A Johnson bar is a long, steel, lever-like device found on a steam railroad locomotive and used to control the train's direction. The origin of the name "Johnson bar" is uncertain, however, many believe it comes from the long, steel pry bar of the same name. Located on the under-workings of the steam locomotive and extending upward into the cab through the floor, the Johnson bar is locked into a slot in the floor to actively engage and lock the locomotive in the correct gear. Although the Johnson bar is used for forward, neutral and reverse gear changes, it is often referred to as a reversing lever.
Steam locomotives use a large system of valves and pressure lines to transfer the steam into linear power and motion. The engineer is able to divert and reverse the flow of steam to the drive pistons by engaging the Johnson bar. The trick in this maneuver is that the Johnson bar must be moved at the precise instant to easily change the direction of the drive pistons, thus reversing the locomotive's drive wheels. Attempting to move the lever at the wrong time will result in grinding and the possible escape of the majority of the steam. This will leave the train immobilized and waiting for the steam pressure to rebuild itself.
In a modern diesel-electric locomotive, a small handle often manufactured from plastic is used to drive and reverse the locomotive's engines. This small handle is correctly named a reverser. The engineer is able to remove this lever when he exits the train, effectively preventing the movement of the train by anyone without a reverser. Unlike the steam engines that used changes in steam direction to alternate gears, the diesel-electric locomotive changes direction in electrical current to change gears. One of the notches in the locking component of the Johnson bar was routinely called the "company notch" due to it being the position that got the utmost in performance from the locomotive.
When an engineer placed the Johnson bar in the company notch, the train was pulling the maximum amount of payload at the best economy for using fuel and steam pressure; in other words, the train was working to the company's advantage. It was the job of the engineer to control the heat in the firebox and the water in the boiler to ensure the very best performance of the locomotive and, therefore, the train. Manipulation of the Johnson bar was one area that a proficient engineer was required to excel in.