The assessment procedure of a 90-years old, nailed steel Italian railway bridge, the Campasso Bridge, is discussed in this paper. The characterization of materials, the evaluation of the corrosion effects on the structural members and the connections are taken into account by means of both in situ and laboratory tests. The identification and calibration of a structural model is then approached both by means of a load test and by comparison of the results given by 1D, 2D and 3D models. On this basis, it is showed that the bridge can be re-opened to heavy commercial railway traffic (D4-type 4 axle carriages, 22.5 tons/axle) provided that the north-rail only is kept in service and few rehabilitation works are performed. The fatigue residual life is estimated not less than 20 years, much longer than the expected service-time left, but it can be reached if the corrosion phenomena are stopped by adequate interventions.