Excessive seismic-induced dislocation of railway simply supported girders can damage or destroy the rail fastener track structure. To investigate the mechanical and deformation behavior of the rail fastener track girder system (RFTGS) under such dislocation, a full-scale specimen was fabricated and subjected to a low-cycle reciprocating loading test. Based on the experimental results, a numerical simulation model was developed to analyze the failure states of the fastener and sleeper. The results indicate that slip occurs during cyclic loading due to the lateral gap between the rail and fastener, leading to significant pinching in the hysteresis curve. When the dislocation displacement reached 60 mm, the concrete at the sleeper shoulder adjacent to the girder gap cracked, with further displacement causing complete shear failure. The steel rail exhibited overall bending deformation without abrupt changes, while the strain within the rail remained significantly below the yield strain. Under normal span conditions, neither the span length nor the dislocation mode significantly influences the dislocation amplitude at which fastener and sleeper failure occurs; however, larger fastener spacing corresponds to a higher girder dislocation threshold. A maximum girder dislocation amplitude of 61 mm can be regarded as a sign of sleeper and fastener failure, providing a basis for seismic design and resilience analysis of railway simply- supported bridges, and by using this lateral dislocation index, engineers can determine whether the fastening system on a railway bridge fails during earthquakes, based on a bridge model without track structures.