Seismic Response Analysis of Concrete Box-Type Subgrade in High-Speed Railways

China is prone to frequent earthquakes, impacting both the construction and operation of high-speed railways. The seismic response analysis is essential for the promotion and structural optimization of novel box-type subgrade structures. This paper develops a nonlinear integrated spatial dynamic model of a ballastless track and box-type subgrade, investigating structural vibrations under multi-dimensional seismic actions using the time–frequency domain method. A seismic performance analysis of the structure is conducted to examine the influence of seismic intensity on its seismic response. Results indicate that the box-type subgrades exhibit good integrity under seismic actions, primarily undergoing rigid body motion, with forces in all components well below design strength, meeting the Chinese seismic fortification requirements. Compared to traditional simply supported beams, box-type subgrades show better adaptability to earthquakes. As seismic intensity increases, vibrations and deformations of box-type subgrades change linearly, while vibrating forces increase nonlinearly.