The modification of old lines is one of important trends of the development of express railways. The dynamic performance of roadbed of railways under the loads of trains is studied in this paper.First

a theoretical model of dynamic response of the track system under moving loads is formulated with the structure of the track system considered as a continuously supported Euler’s beam to implement its numerical analysis through the secondary development of the ANSYS software;then

a numerical model of the rectilinear segment of the track system is established to analyze the influence

of the speeds and intensity of the moving loads

the length and depth of the dis-glide wave

on the vertical displacement and acceleration of the steel rails and rail sleepers when the old line is speeded up to 200 km/h.The results in this paper will provide a technical reference for the formulation of new code of railway protection.