It is very important to understand the mechanism of track deterioration and predict track settlement or track irregularity growth rate for reducing track maintenance costs and designing new track structures. The authors established a track settlement prediction model which consists of a vehicle/track dynamic model and a track settlement law. The adequacy of the prediction model was roughly validated comparing with track site measurements carried out over five years. The track settlement law was established based on ballast settlement experiments in which applied loads were simple but practical loading conditions are not so simple. In this paper, the effect of the distance between leading and trailing axles of a bogie on the interaction between the responses, such as rail seat force and sleeper displacement, excited by the leading and trailing axles has been studied with the focus placed on vehicle speed, unsprung mass, the stiffness of track structures and rail type.