Mechanics-based reconstruction of post-derailment path evolution in a seismically excited high-speed train–track–bridge system

Earthquake-induced post-derailment dynamics remain poorly understood because conventional running-safety indices lose physical meaning once a wheel persistently leaves the rail-head support corridor. This study develops a mechanics-based event-reconstruction framework linking persistent wheel–rail support loss to first structural-boundary arrival. Wheel envelopes are reconstructed from axlebox and bogie kinematics in a bridge-following frame, and stage-specific geometric and kinematic criteria identify support loss, slab contact, post-slab reversal and boundary arrival. The framework is applied to a three-dimensional train–track–bridge finite-element model validated using retained-contact wheel–rail and reinforced-concrete slab impact tests, and evaluated for 270 seismic scenarios comprising 30 ground motions, three train speeds and three peak-ground-acceleration levels. Wheel lift preceded persistent support loss in 257 of 260 complete chains, yet both events occurred on the same side in only 45 of 259 observable cases. The loss-to-slab interval retained coupled lateral–yaw motion, whereas first slab contact produced a selective roll-dominant response rather than a complete kinematic reset. Of 40 boundary arrivals opposite to the transverse-velocity direction at support loss, 39 exhibited sustained post-slab reversal. The first boundary arrival was predominantly non-local: only 21 of 204 cases were controlled by the first-loss axle. Across 360 adjacent one-factor comparisons, increasing speed or peak acceleration could preserve or alter path status, showing that inputs condition rather than uniquely determine path development. The framework therefore recasts post-derailment behavior as successive constraint transitions, motion-state inheritance and component-envelope competition within the modeled two-car formation.