A realistic and economical computational assessment of the dynamic behaviour of railway bridges requires, first and foremost, input parameters that correspond to reality. In this context, the computationally applied damping properties of the structure have a decisive influence on the results in the prediction of resonance effects. Concerning the damping factors used in dynamic calculations, the EN 1991-2 standard prescribes damping factors depending on the type of structure and the span. However, these factors can be regarded as very conservative values. As a result, in-situ measurements on the structure are often necessary to classify a bridge categorised as critical in prior calculations as non-critical. Based on extensive experimental research at TU Wien into the energy dissipation mechanisms in railway bridges with a focus on the dynamic behaviour of the ballast superstructure, it was able to derive an approach to determine the damping factor of railway bridges by calculation for the first time. The basic idea in the mathematical determination of the damping factor is to define and superpose the dissipative contributions of the supporting structure and ballast superstructure separately. Initial comparisons with measured damping factors of existing structures reveal significant potential for generating realistic damping factors as a basis for dynamic assessments of railway bridges using the new approach for mathematical calculation of the damping factor. This contribution presents a further measurement-data-based verification and validation of the derived mathematical approach. An extensive measurement campaign on existing railway bridges in the Austrian railway network serves as the basis, whereby new evaluation methods to determine realistic damping factors from in-situ measurements are also applied. In addition to the dynamic parameters identified from measurement data, the measured deformation state due to dynamic excitation is also used to validate the mathematical approach. The results show potential for a realistic dynamic assessment of railway bridges, which can minimise the discrepancy between measurement and calculation and offer possibilities for predictive maintenance.