What is "bank engine"

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.