For ballasted tracks, considerable maintenance work is needed to rail joints with fish plates and rail welds. In particular, loose sleepers which appear on the way of progressing vertical track irregularity are very important to understand track deterioration including track settlement and fatigue of track materials. Track dynamic behaviours excited by loose sleepers can be predicted to some extent with a vehicle/track dynamic model constructed at RTRI which was validated with early experiments at a track site. Estimating the bending fatigue life of rail welds supported by loose sleepers based on the analytical results carried out with the dynamic model, it was clearly understood that loose sleepers had great influence on the bending fatigue life of rail welds.