Rail steels require sufficient hardness to provide resistance to wear from contact with the train wheels and it must also be resistant to changes in rail profile from plastic deformation and to cracking caused by rolling contact fatigue as shown below.
Rolling contact fatigue damage in rails
The microstructures of rail materials changed through the years as cast iron was replaced by wrought iron and then various grades of steel as shown below.
The vast majority of rail steels in use today contain about 0.7-0.9% carbon and have a microstructure known as pearlite consisting of very fine alternating layers iron and cementite (Fe3C). However higher hardness rails have bainitic or martensitic microstructures.