Abstract: To simplify the calculations of wheel-rail contact force and governing equations,a surface crack problem in a semi-infinite space was reduced to solving a set of hyper-singular integral equations with displacement jumps as unknown functions.The boundary element method was used in numerical analyses to disperse the integral equations,and a special numerical treatment for the hyper-singular integrals was proposed.Finally,the surface crack problem in a semi-infinite space with or without internal oil infiltration between crack surfaces under rolling load was discussed.The research result shows that oil infiltration will accelerate the propagation of surface cracks.