This data set is obtained from a monitoring campaign of 15 months on a steel bowstring railway bridge in Leuven, Belgium. In this period, the connections of the diagonals to the bridge deck and the arches were strengthened after the observation of damage. The monitoring consists of measurements of acceleration on the bridge deck and the arches, strain on the bridge deck and the diagonals connecting the bridge deck with the arches, strain on the rails, displacement at the bearings, and temperature and relative humidity. Data is provided for two train passages and six periods of ambient vibration per day. In addition, the evolution of the identified modal characteristics over time is added. The data set is well-suited for the validation of vibration-based structural health monitoring methods, as it includes data for different states of the structure, i.e. before, during, and after the retrofitting. ; Kristof Maes is a postdoctoral fellow of the Research Foundation Flanders (FWO), Belgium (grant number 12Q9218N). FWO also provided additional funding for the measurements by means of research grant 1511719N. The financial support by FWO is gratefully acknowledged.