Old Steel railway bridges were not built for the current trafic intensities and are therefore prone to fatigue damage. Two old truss steel railway bridges have been considered for the assessment of their steel members' mechanics damage. Although the ageing of the bridges, their importance on the railway lines encouraged the challenge to study the cases and find out the solution that ensures the safety of damaged members. The damaged members represent fortunately  less than 25% of the bridge elements. Three approaches have been performed for the assessment of the causes and remedies for the phenomena, consisting of complete surveying for the existing bridges, complete structural modelling and stress analysis under real train loads, and field tests through non-destructive tests and static load tests. The stress analysis has been carried out with a simplified FEM model, calibrated with observed strains derived from real scale testing result.  The results lead to two mechanics damage for members, the first is related to the fatigue resulted from the increasing service loads, speeds, and cyclic stresses. While the second is related to the tension buckling resulted from the reduction of member thicknesses, owing to flanges sections pack rust, and causing them over time to locally buckle. The paper investigates the two mechanisms, estimation of the bridge remaining lifetime, the cyclic stresses and the tension buckling instability. The practical solutions for the rehabilitation of damaged member have been presented