When a train enters a bridge, passenger sitting inside will feel a sudden bump in the track, which not only affect the riding comfort of the passengers but also put a dynamic impact on the bridge structure. Due to this impact force, we have very serious maintenance problems in the track close to the bridge structure. This sudden bump is produced when train travelling on the track suddenly hit by a very stiff medium like bridge structure. In order to reduce this effect, transition zones are introduced before the bridge so that the change in stiffness will occur gradually without producing any bump.   This master thesis examine the effect of track stiffness on the bridge dynamic response under different train speeds from 150 to 350 km/h with interval 5 km/h and also estimate the minimum length of transition zones require to reduce the effect of change in stiffness on the bridge. Study also gives us some guidelines about the choice of loading model of the train, location of maximum vertical acceleration, effect of ballast model on the results and minimum length of transition zone needs to include in the bridge-track FE-model, for dynamic analysis of the concrete bridges. To carry out this research MATLAB is used to produce an input file for the ABAQUS FEM program. ABAQUS will first read this file, model the bridge and then analysis the bridge. MATLAB will again read the result file, process the result data and plot the necessary graphs.   The Swedish X2000 train is used for this study, which has been modeled with two different methods: moving load model and sprung mass model, in order to see the difference in results. For verification of the MATLAB-ABAQUS model, a 42m long bridge is analysed and results are compared with known results. In this study, concrete simply supported bridges with spans of 5, 10, 15, 20, 25 m have been analysed.