In rail terminology, hammer blow or dynamic augment is a vertical force[1] which alternately adds to and subtracts from the locomotive's weight on a wheel. It is transferred to the track by the driving wheels[2] of many steam locomotives. It is an out-of-balance force on the wheel (known as overbalance[3]). It is the result of a compromise when a locomotive's wheels are unbalanced to off-set horizontal reciprocating masses, such as connecting rods and pistons, to improve the ride. The hammer blow may cause damage to the locomotive and track if the wheel/rail force is high enough.