A realistic and economical dynamic assessment of railway bridges requires, first and foremost, input parameters that correspond to reality. In this context, the applied damping properties of the structure have a decisive influence on the results in the prediction of resonance effects and further in the assessment of the compatibility between rolling stock and railway bridges. Concerning the damping factors used in dynamic calculations, the standard prescribes damping factors depending on the type of structure and the span. However, these factors can be regarded as very conservative values which do not represent reality. As a result, in-situ measurements on the structure are often necessary to classify a bridge categorized as critical in prior dynamic calculations as non-critical. Regarding in-situ tests, a measurement-based determination of the damping factor is inevitably accompanied by a scattering of the generated results due to the measurement method used and also as a result of the individual scope of action of the person evaluating the test and this person’s interpretation of the measurement data. With this background, this contribution presents new methods and analysis tools for determining the damping factor, intending to reduce the scatter of the results and limiting the scope of action of the person evaluating the test. Methods and analysis tools are discussed for methods in the time and frequency domain. Based on in-situ tests on 15 existing railway bridges, the data-based procedure for determining the damping factor is explained, and the methods are compared in the time and frequency domain. It is shown that a clearly defined evaluation algorithm can significantly reduce the scattering of results. Furthermore, it is shown that the excitation method substantially influences the determined damping factors.