The damage of railway lines during heavy haul freight train passage is closely related to the stress state of the subgrade surface.In order to study the stress path in subgrade soils under moving trains,this study utilized a dynamic freight-railway model to analyze such stress on during freight train operations.The study investigated the effects of ballast layer thickness,axle load,speed,and other factors contributing to subgrade surface damage.The extent of subgrade stress and related damage were obtained using the Euler beam model to analyze the elastic half-space under one moving load.The stress path in soils under the moving load was analyzed.Based on the modeling,it was determined that the stress state changes from pure shear to triaxial shear and back to pure shear in one cycle.According to the stress path curves for different moving speeds,it was determined that,when the moving load is high,the horizontal shear stress increases dramatically.The results further indicate that the stress state changes from the initial state to pure shear,to triaxial shear,and back to pure shear.The principle axis of stress rotates 180°.However,it is more complex due to the interaction of the wheels.A ballast layer deeper than 0.5 m,train speeds exceeding 70 km/h,the modulus of the subgrade materials less than 160 MPa,and an axle load higher than 27 t,all of these factors may result in the plastic deformation within the subgrade surface.If the stress path reaches the failure line,the hypothesis of elasticity will be invalid.