Rail wears due to the tangential forces which occur when steel wheels roll over steel rails. In principal wear rates can be relatively low (in straight track rail head wear of 0.05-0.1mm per 10 MGT is not unusual) but when forces increase, for example when trains negotiate curves, the wear could be an order of magnitude greater. Especially wear on the gauge face of the rail can be significanty high on rails in curves.

On tangent/straight track the head of both rails slighty flattens and plastic flow occurs towards the field and gauge side of the rail

On curved track(medium to tight curves) the high rail typically wears to the shape of the wheel in the flange area. This type of wear is often referred to as side wear. The low rail in curves tends to flatten due to plastic flow

In both cases where the shape of rail changes, either due to side wear or plastic flow, the level deformation depends on the level of friction between wheels and rails. Adequate lubrication, either to rails or wheels is often able to control friction thus controlling level of wear and plastic deformation of rails.