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Some typical fatigue failures of butt-welded rails consist in fractures ofthe web. In the early stage, crack propagates parallel to rail surface but, after a while,it tends to propagate with a slant surface. The aim of the present work is to analyse themechanisms involved in crack propagation in rail’s web in order to be able to predictthe crack path and, in a second step, the rail inspection intervals. From this point ofview, FEM analyses of an observed fracture were carried out in order to determinate K Iand KII history at different positions of crack tip during the passages of the typical loadsinduced by trains. The results have shown that the initial flaw tends to follow a pathwhere ∆KII is close to maximum values and a small superimposed KI traction is present.Fractographic analyses have confirmed that propagation is mainly controlled by modeII and that mode I is prevalent only after the final bifurcation of the cracks.