Rolling contact fatigue and bending fatigue of rail welds are very important factors to understand rail failures. The estimation method of service life of rails has been developed and improved at Railway Technical Research Institute. On the other hand, the authors established a track dynamic model to understand track dynamic behaviours for the design of track and vehicle components. Basically, rail bending stress distributions measured at track site have been adopted to estimate the fatigue damage of rail welds. However, the bending stresses can be recently predicted presicely enough with the track dynamic model. The authors adopted the dynamic model to obtain various rail bending stress distributions for the prediction of service life of rail welds. As a result, the influence of track stiffness on the bending fatigue of rail welds was made clear.