In railroad tracks, the wearing down, or battering, of rail ends is caused mainly by the cold flow of metal. When trains pass over a rail, the concentrated load applied under the wheels produces at times a stress greater than the elastic limit of the steel in the rail.
This stress is further increased by the hammer-like blows resulting from any unevenness in the height of the abutting rails, or poor joint or surface maintenance. In addition, the metal at the ends of the rail can flow in two directions, laterally and longitudinally. This causes a much more rapid lowering of the surface of the rail at the very ends.
In the past, rails were heat treated to raise the elastic limit of the steel in the tread portion of the rail sufficiently to overcome the cold flow effect. The oxy-acetylene process was used. Heat treating was accomplished by heating the end portions of the tread surface of the rails until they were well above the transformation point, then quenching the rail ends. If necessary, a second heat treatment was applied to obtain the required degree of hardness, about 400 on the Brinell hardness scale.
The advent of rail joint welding almost completely did away with the need for rail-end hardening.