A realistic and economical dynamic assessment of railway bridges requires accurate input parameters in mechanical. In this context, the applied damping characteristics significantly influence the accuracy of resonance effects predictions and the assessment of the compatibility between rolling stock and railway bridges. However, the EN 1991-2 standard prescribes highly conservative damping factors that do not represent reality. Consequently, in-situ measurements are frequently necessary to reclassify a bridge deemed critical in preliminary dynamic calculations. Regarding in-situ tests, measurement-based damping factors are inevitably accompanied by a scattering of the results due to the measurement method used, the individual scope of action of the person evaluating the test and the individual interpretation of the measurement data. This contribution presents a novel evaluation method for determining the damping factor based on in-situ measurements. The objective is to develop an easily applicable method that yields reliable and beneficial high damping factors while intending to reduce the scatter of the results and limit the scope of action of the person evaluating the test. The method is applied in a measurement campaign on 15 existing railway bridges, where it is shown that a clearly defined evaluation algorithm can significantly reduce the scattering of results. In addition, an approach is presented that makes it possible to determine the damping of railway bridges by calculation without the need for in-situ measurements. This first-time possible mathematical determination of damping represents an alternative to the overly conservative standard and enables a realistic assessment of the dynamic behaviour of railway bridges.