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 circum-stances 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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