Dynamic Response of Vehicle–Track–Bridge Coupled Systems with Pad Gaps in Fastening Systems

The dynamic performance and reliability of fastening system components have become increasingly important for ensuring train operation safety. However, a persistent issue in service is the formation of gaps beneath rail pads, which alters the stiffness distribution of the fastening system. This study investigates the impact of pad gaps on the dynamic response of vehicle–track–bridge coupled systems. A spatial mechanical model of the fastening system incorporating pad gaps was developed. On this basis, the vehicle–track–bridge coupled model was established using the finite element and multibody dynamics methods, and subsequently validated. The influence of train speed, the number of gap pads, and gap height on the system’s dynamic response were systematically analyzed. The results indicate that pad gaps significantly modify the system’s dynamic behavior, increasing rail displacement by up to 28.7% and causing excessive rail and bridge vibrations. Higher train speeds further amplify system responses, with rail vertical displacement and wheel–rail vertical force increasing by 68.4% and 35.2%, respectively. Increasing the number of gap pads intensifies rail and bridge vertical displacements, and nonlinearly amplifies the wheel–rail vertical force when the number exceeds three. In addition, larger pad gap heights aggravate rail vertical displacement and wheel–rail vertical force by 60.5% and 28.7%, respectively, while inducing complex vibration characteristics due to local stiffness degradation.