The fast movement associated with high speed trains can cause significant dynamic effects within the supporting railway track structure. The speed at which maximum dynamic re-sponse occurs is known as the 'critical velocity' and is undesirable because large rail vibra-tions are generated when travelling close to it. These vibrations can cause a safety concern, and also propagate to the free-field where they disturb nearby buildings. A method to mini-mise these vibrations is to stiffen the soil directly below the track either via soil replacement or soil improvement, however both options are expensive. Their cost can be reduced though if either the depth or stiffness magnitude of the replacement is optimised. Therefore this work develops a track-ground model using the thin-layer method, which is capable of as-sessing the effect of different combinations of soil improvement on track vibration levels. It is shown that if improvement is carefully designed, performance can be maximised for mini-mum cost. Similarly, if improvement is poorly chosen, it can result in marginal improvement, and in some cases even amplify track vibration.
