A beam-rail-pier 3D finite element model was built by adopting the mechanism of ideal elastic-plastic ballast resistance under the background of a 156m long-span simply supported steel truss bridge on Huangling-Hancheng-Houma Railway line. The purpose was to study the distribution of longitudinal force due to temperature variations, bridge deflection, braking force and the breaking force of long rail in steel truss bridges. The influences of design parameters on longitudinal forces, such as the bearing arrangements of adjacent simply supported bridges, pier longitudinal stiffness and the arrangement of small resistance fasteners, were also discussed. It has been shown that the longitudinal force due to temperature variations plays a major role. Secondly, the bearing of adjacent simply supported bridge should be in the same direction as the steel truss bridge. Thirdly, the longitudinal force due to temperature variations and structural deflection increases and the braking force decreases as the pier longitudinal stiffness increases. Finally, the small resistance fasteners have a good effect on the longitudinal force due to temperature variations, which can reduce the force by 36% after being installed.