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 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 addition of extra weights on the wheels reduces the unbalanced reciprocating forces on the locomotive but causes it to be out of balance vertically creating hammer blow.
Additional balance weight — "overbalance" — can be added to reduce this, typically enough to "average out" the vibrations by make the remaining forces and moments equal in the vertical and horizontal directions.
One solution to this was the duplex locomotive, which spread the driving power over multiple sets of pistons, thus greatly reducing hammer blow.
Steam turbine locomotives lack pistons, valve gear and other fore-aft reciprocating components making it possible to balance the wheels and connecting rods to eliminate hammer blow.
Whilst many of these turbine locomotives suffered problems in service (usually excessive fuel consumption and/or poor reliability) they did prove to be free from hammer blow and offered a way of achieving high power outputs and speeds without causing track damage.