Effect of Near-Fault Vertical Ground Motion on Seismic Response and Damage of High-Speed Railway Isolated Track-Bridge System

China's high-speed railway (HSR) network extensively utilizes bridge structures to maintain track regularity, with numerous lines extending across active fault zones that present substantial seismic hazards. Near-fault ground motions exhibit distinctive characteristics, particularly strong vertical components, that differ significantly from far-field earthquakes. Seismic isolation design is essential for improving the seismic performance of near-fault HSR track-bridge systems, but research on the influence of vertical ground motions (VGMs) on their dynamic response and damage mechanisms remains inadequate. To address this, this study developed a refined finite element model (FEM) in OpenSEES incorporating CRTS II slab ballastless tracks, bridges, and friction pendulum bearings. Nonlinear time-history analyses were conducted to investigate structural responses and damage degrees under varying ratios of vertical-to-horizontal peak ground acceleration (αVH) and seismic intensities (frequent, design, and rare earthquakes). The results demonstrate that αVH values in near-fault regions are densely distributed between 0.5–1.5, exceeding the values specified in the design codes. The impact of VGMs intensifies with seismic intensity: negligible under frequent earthquakes but significantly amplifying damage to piers, bearings, and track interlayer components (e.g., sliding layers and CA mortar layers) during design and rare earthquakes. While seismic isolation effectively mitigates structural responses through energy dissipation by bearings, it may increase sliding layer displacements and lead to bearing failure under rare earthquakes. Based on damage control and economic considerations, tiered αVH recommendations are proposed: 0.67 for frequent, 0.9 for design, and 1.2 for rare earthquakes. These findings provide critical references for the seismic design of HSR infrastructure in near-fault regions.