This paper present experimental testing of an in-service railway bridge where viscous dampers were installed near the roller support, with the objective to mitigate vibrations from passing trains. Forced vibration tests were performed before and after installing the dampers, which showed a significant increase in resulting modal damping. A long-term monitoring system was installed and more than 4000 train passages has been recorded to date. The system records both the response of the bridge and the resulting force in the dampers. Based on a Bridge Weigh-in-Motion system (BWIM), the train speed and train types are identified. The estimated train speed varies from 40 to 160 km/h for passenger trains, which enables to study the dynamic performance of the bridge under different loading frequencies. The variation in modal properties, e.g. natural frequency and damping is studied as function of the vibration amplitude. The bridge is located in the North of Sweden where the temperature is expected to range from +30°C in the summer to -30°C in the winter. Experimental data will be collected to further study any seasonal changes on the dynamic properties of the bridge, e.g. due to longitudinal stiffness of the continuous track system.