The present paper aims to study the influence of the soil/structure interaction on the dynamic behaviour of bridges, when traveled by high speed trains. The article comprises the study of two distinct bridges, in order to better understand the importance of this interaction. Different models with soil inclusion are created in order to study, comparatively to the model with stiff supports, the results for accelerations, displacements and section forces of the deck. The study evidences the aggravation in the dynamic behaviour of the bridge when soil is considered, hence the importance of its simulation. Accelerations in models where soil was considered are always higher than in the model with stiff supports, being verified that some are not in compliance with the upper limit of 3,5 m/s2, proposed by the Eurocode. In addition, section forces are always more severe in those models as well. A comparison of the models eigenvalues revealed the correct consideration of soil stiffness, and a dependance between the modes of vibration and the effects on the structures behaviour was verified. This article suggests a way to consider soil simulation in rail bridge design.