The book outlines the current understanding of the formation of structural and phase states and defect substructure rails of different categories, subjected to differentiated and bulk quenching. Methods of modern materials science and, above all, transmission electron microscopy, are used for stratified analysis of rail categories: low-temperature reliability, enhanced wear resistance and contact fatigue strength, highest quality after bulk quenching and tempering and differentially hardened by various regimes. The quantitative parameters of the dislocation substructure, internal stress fields and structural and phase states, formed by diffusion and shear mechanisms of the gamma to alpha transformation in the rail head on the central axis are determined.