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A bank engine (United Kingdom/Australia) (colloquially a banker), banking engine, helper engine or pusher engine (North America) is a railway locomotive that temporarily assists a train that requires additional Engine power or tractive effort to climb a gradient (or bank). Helpers/bankers are most commonly found in mountain divisions (called "helper districts" in the United States), where the ruling grade may demand the use of substantially greater motive power than that required for other grades within the division.
Bankers or helpers were historically positioned at the rear of the train, in which case they also protected against Freight car 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 Rail siding or stub so as to clear the mainline and get ready for the next train. A common practice with Knuckle coupler was to remove the knuckle from the front Railway coupling. 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.
Special heavily constructed were sometimes used in helper areas. Ordinary cabooses were built as lightly as practical and might be crushed by the helper/pusher's force, which could be as much as 90 tons. The heavy cabooses allowed crews to avoid the time-consuming procedure of splitting the train just ahead of the caboose. photo of reinforced outside braced caboose.
Pushers/helpers were commonly designed to provide extreme power for very short runs; as a result they could not push at full power for very far before steam pressure dropped. If it could push enough to get the train to the top of the grade, then it could build up pressure while coasting back down and while waiting for the next train to come along. This practice was common in Europe.
Since it was not possible to remotely control a steam locomotive, each helper had to have a full crew on board. Careful coordination was required between engine crews to assure that all locomotives were operated in a consistent manner. Standard Train whistle were employed to tell the helper crew when to apply power, drift or brake. A misunderstanding of signals by a pusher locomotive crew could result in a major wreck if the lead locomotive applied brakes while the bank engine was still applying power. The usual result was that the train would experience a violent run-in (an abrupt bunching of train slack), resulting in the derailment of part or all of the train.
The town of Helper, Utah, was named after these engines. It was where helper engines were kept to assist on the climb to Soldier Summit.
Adding locomotives in the middle of the train has the distinct advantage of applying the helper power to only part of the train, thus limiting the maximum drawbar pull applied to the first car of the train to a safe level. The narrow Track gauge portions of the Denver and Rio Grande Western Railroad, in particular, used "swing helpers", which meant the helper locomotives were placed mid-train at a point where they were pushing and pulling an approximately equal amount of tonnage, said location being referred to as the train's "swing point". This was also done to balance out the "slack" in the train between the locomotives, the swing helpers, and the end train helpers just in front of the caboose. However, this arrangement requires splitting the train in order to add or remove the helper engine(s), which can be a time-consuming maneuver. However, on some American railroads it was necessary to an extent, because operating rules required end of train helpers to be added at the end of the train, but in front of the caboose. This was done for the safety of the train crew riding inside the caboose.
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