Helper locomotive

A bank engine (United Kingdom/Australia) (colloquially a banker) or helper engine or pusher engine (North America) is a railway locomotive that temporarily assists a train that requires additional power or traction to climb a gradient (or bank). Helpers/bankers are most commonly found in mountain divisions (called "helper districts" in the U.S.), where the ruling grade may demand the use of substantially greater motive power than that required for other grades within the division.

Helpers/bankers were most widely used during the age of steam, especially in the American West, where significant grades are common and trains are long. The development of advanced braking systems and diesel-electric or electric locomotives has eliminated the everyday need for bankers/helpers in all but a few locations. With the advent of dynamic brakes on electric or diesel-electric locomotives, helpers/bankers can also be used to provide more braking force on long downhill gradients.

Bankers or helpers were historically positioned at the rear of the train, in which case they also protected against wagons or coaches breaking away from the train and running back downhill. Also, in a pusher role, it was possible for the helper/banker to easily separate once the train had crested the grade. Once separated, the banker would return to a siding or stub so as to clear the mainline and get ready for the next train. A common practice with knuckle couplers was to remove the knuckle from the front coupler. The locomotive would be brought up behind the last car of the train while the train was moving slowly. The air brake hose would not be coupled. When the train no longer required assistance, the helper/pusher would slow, then reverse and coast back down the grade to its siding at the bottom of the grade. This practice was outlawed in North America after the end of the steam era.

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