The travelling of high-speed trains on ballasted tracks creates geometrical disorders, which are found to evolve fast with the traffic loading as compared to the disorders observed on usual speed rail tracks. These high-speed rail lines thus require frequent maintenance associated to high direct and indirect costs. The circumstances leading to such disorders are not well understood and must be studied to allow the definition of design methods that minimize them. Among the factors that may favor the development of geometrical disorders, inertia forces within the railway structure are suspected to play an important role. In that context, this paper presents a method to compute the dynamic response of the structure under moving loads and thus to analyze the combined effects of stress and acceleration on the ballast layer. The developed method is based on continuum mechanics and relies on the decomposition into loading waves of the distribution of vertical stresses at the surface of the medium. This distribution results from loads applied on the rails and is computed by consideration of the system made up of the rails, the sleepers and the structure. The global response is obtained by recombination of the contributions of the loading waves, each of which moves at a different speed. This method is implemented in a numerical program which is used to study the dynamic response of a three-layer structure. It is shown that the influence of the load speed on the deflection and the vertical stress within the ballast layer is relatively small by comparison to that on the vertical acceleration. In some parts of the structure, small vertical stresses are associated to significant vertical accelerations oriented downward. This combination is suspected to promote the development of instabilities within the ballast layer.
