Cracks that begin at the surface of railway track do so in a layer of metal which has undergone severe plastic deformation. The cracks grow at a shallow angle to the surface. They may then turn upwards resulting in a spall of metal leaving the rail. Alternatively, they may branch downwards to form a transverse crack in the rail. This picture reflects the complexity of the stress state which the rail effectively moves through as the wheel passes overhead. The operating circumstances vary with time, notably with the presence of fluids, rain water, snow and accelerations and decelerations of trains. Thus a long crack will propagate by more than one mechanism as it moves downwards from the plastically deformed surface layer into a predominantly elastically loaded region. Within each region there are competing mechanisms for crack growth and branching. This paper attempts to explain this scenario.
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