White Etching Layers (WELs) were found to form on different rail grades in the railway networks across the world. The presentence of WELs on the rail promotes rolling contact fatigue defects, which is detrimental to railway transportation safety. However, the formation and evolution of WELs on different rail grades are not fully understood, as is their contribution to the formation of RCF defects on different rail grades. The extensive field monitoring, laboratory experiment, and material characterization were carried out to systematically investigate the formation mechanism and evolution of WELs on Standard Carbon (SC) rail and Head Hardened (HH) rail, which are widely used for passenger and freight transportation. The long-term field monitoring provides evidence that the formation and distribution of WELs are influenced by repeated rolling contact during the service, and HH rail presents a susceptibility to form WELs more than SC rail. The characterization of thermally induced WELs introduced by the laboratory Gas Tungsten Arc Welding (GTAW) experiment indicates that WELs formed on HH rail lead in size and hardness compared to SC rail. Both the WELs' microstructures on SC and HH rails exhibit the ultra-fine plate-like martensite. However, there are isolated islands of undissolved pearlite in the WELs on SC rail, while WELs on HH rail present remnants of cementite and trace of the lamellar feature. The formation kinetics of WELs on different rail grades is significantly different due to the interlamellar variation in SC and HH rails, and the difference between the austenite growth rate and dissolution rate of cementite.