Fatigue damage behavior and structural optimisation of asphalt concrete-reinforced subgrade in ballastless tracks under extreme climates

In cold regions, extreme cold and freeze–thaw cycles cause frost heave, layer separation, and fatigue cracks in high-speed railway ballastless tracks, posing a risk to the structure and safety. This study examines CRTS III tracks with full-scale fatigue tests at − 48 °C and freeze–thaw cycles (− 50 °C to 45.8 °C), comparing three subgrade structures (with and without asphalt layers of varying thickness). A track-subgrade finite element model evaluates the impacts of asphalt thicknesses (7, 8, 10, 12 cm) on fatigue damage. The asphalt layer greatly boosts cold elastic stability, resists plastic deformation, and curbs frost heave; benefits rise with thickness. Fatigue damage mainly comes from tensile stress caused by train traction. Increasing asphalt thickness boosts fatigue resistance, but gains shrink beyond 10 cm: raising thickness from 10 to 12 cm lowers damage by just 0.83%. Balancing deformation, fatigue, and cost, 10 cm is the best asphalt layer thickness. This study offers a quantitative basis for safe ballastless track design in cold regions.