Polymer sleepers can combine a stiffness behaviour comparable to that of timber sleepers with the consistency and lifespan of concrete sleepers. Their specific characteristics and potential advantages should be considered, the most prominent being a possible low bending stiffness, material viscoelasticity, a possible anisotropic sleeper composition, and freedom of shape. To determine in-track response of polymer sleepers, displacement measurements were taken for polymer sleepers and for reference concrete sleepers on the track.A polymer sleeper should be considered a Timoshenko beam on an elastic foundation, owing to the possible low bending stiffness of polymer sleepers and their possible anisotropic composition. To accommodate such an analysis, a simplified calculation method was derived, using a sleeper flexibility factor to account for sleeper bending and shear stiffness effects on track deflections. Static tests on a rubber bed (representing ballast) were performed as a validation of this simplified calculation method.Laboratory testing must be performed dynamically, at strain rates applicable to in-track conditions, because of viscoelasticity. Repeated load tests were performed intermittently, introducing pauses between numbers of load cycles. This reduced heating and creep effects and has been shown to give results more representative of the track.As a result of repeated train loads, a gap arises between sleeper and ballast at the rail seat location, and the sleeper shapes according to this gap (i.e. beds-in) on every train passage. The bedding-in process ends when contact stresses between the sleeper and ballast are equalised over the sleeper length. An iterative, numerical calculation method was developed to predict the formation of these gaps and to optimise sleeper shapes for this support condition. Optimisation calculations and scaled ballast tests were performed for several sleeper shapes and depths, and they show that balancing of the sleeper (preventing it from becoming centre-bound or end-bound) and concentrating the sleeper-ballast contact area around the rail seat can reduce resilient displacements by up to 40% without increasing material usage.The scaled ballast test also shows that overall settlement is related mainly to the ballast surcharge above the sleeper bottom. As ballast is normally level with the top surface of the sleeper, the sleeper depth governs overall ballast settlement.