Trains have been operating on ballasted tracks for over 150 years. During this time demands from the railway organisations and from the industry to increase axle loads and train speeds on the existing railway lines have increased. The reasons for this are both economial and environmental.

Both higher axle loads and train speeds will increase the deterioration rate of the track and cause permanent settlements in the ballast and the underlaying soil. When the track quality decreases beyond certain limits maintenance for relevelling and realignment of the track is necessary, and this is expensive.

The aim of the work presented in this thesis, and of the work performed within the EU project SUstained PErformance of Railway TRACK (SUPERTRACK), is to investigate the dynamic train/track interaction and its effect on track degradation. Less degradation will reduce the maintenance costs regarding track settlement.

The work presented in this thesis focuses on different factors affecting the differential track settlement. By using the finite element method, the dynamic behaviour of a railway track subjected to a moving load has been studied. Dynamic effects of unsupported sleepers and their effect on track settlement have been investigated. Also track stiffness irregularities along the track have been studied, and an optimazation of the track stiffness at a transition zone has been performed.