Regarding the design of longer railway bridges, the track-bridge-interaction plays a decisive role because of the high braking and acceleration forces as well as the connection of the rail with the superstructure. A fundamental parameter by calculating the effects of track-bridge-interaction is the longitudinal stiffness of piers and abutments with horizontally fixed bearings. The horizontal equivalent stiffness is commonly calculated by a pile grillage model. In the case of the Itz valley railway viaduct, the stiffness has been determined experimentally by a static diagnostic load test to allow the verification of the additional rail stresses and the bearing forces with verified input parameters. Associated with the experiments, numerical analysis of the foundation in a 3D FE model of the soil has been carried out. In this article the design, execution and evaluation of the experiment will be presented. Subsequently, the results of the test will be compared with the calculated prediction of the evaluated models.