A lumped mass model of a railway truss bridge is developed. The model considers only the vertical degree of freedom of each truss joint. The vehicle system is idealized as a three degree of freedom model consisting of the carbody and wheel-axle sets. Dynamic interaction equations for the bridge-vehicle system are derived and solved using the numerical integration method. Impact factors for member forces and nodal deflections are generated under the action of a single or a series of three moving vehicles. Finally, a limited parametric study is performed to determine the influence of vehicle speed, vehicle suspension characteristics and sprung mass on impact factors./Author/