Steel truss bridges are typically composed of a large number of slender members and represent one of the primary structural forms of railway bridges, facing the threat of overall collapse caused by explosions from unmanned aerial vehicles. Numerical simulation analysis was conducted on the failure mode and residual bearing capacity degradation law of railway steel truss bridges subjected to contact explosions. Firstly, the reliability of the numerical simulation method was verified by existing explosion tests on stiffened steel plates and steel box arches, as well as the residual bearing capacity of I-shaped steel column after explosion. Subsequently, mesh sensitivity analyses were performed for the damage and failure of upper chord member under contact explosions and for the residual bearing capacity of the entire bridge. Then, the most critical member of the bridge was identified by evaluating its residual performance under 100 kg TNT equivalent explosions at different locations. Furthermore, the variation of the residual bearing capacity with explosion yield was investigated. Finally, the evolution mechanism of damage and failure of the entire bridge under multi-point explosions was discussed. The results show as follows. (1) Under contact explosions, steel truss girder bridges are mainly characterized by localized member damage. For an explosive charge of 100 kg, the overall bridge bearing capacity decreases by 29.8% and 18.0% when the explosion occurs on the side and top surfaces of the upper chord member. The side explosion on the upper chord is the most unfavorable scenario. (2) As the charge weight for side explosion on upper chord increases from 25 kg to 150 kg, the reduction in residual bearing capacity of the entire bridge increases from 8.8% to 33.4%. Taking the ratio of bearing capacity loss to the ultimate bearing capacity of intact bridge as the damage index, a quantitative relationship between the entire bridge damage index and the explosive charge weight is established. (3) Under multi-point explosion scenarios, the damage factor increases to 0.452, indicating the structural redundancy and residual bearing capacity are significantly reduced compared with those under single-point explosion conditions.