This paper deals with the dynamic response of a truss bridge under a single moving train. A planar finite element model of a train-bridge system is used. The train is divided into one-foot sprung masses. The bridge is modelled as frame and truss elements. Vertical random track irregularities are generated by power spectrum density functions. The dynamic equations of motion of the train and the bridge are coupled by the shape constraints and contact forces. The interaction dynamic equation of motion is solved by the Newmark-beta and Newton-Raphson method. The dynamic response of a steel truss bridge with track irregularities under a KTX train is calculated as an example. The dynamic impact factors for different bridge nodes are evaluated and compared with different train speeds and track irregularities conditions.