To analyze the variation in the fatigue path and the stress intensity factor (SIF) of three-dimensional surface cracks in rails under wheel–rail interaction, and to investigate the underlying damage mechanisms leading to rail block spalling. Initially, the SIF at the crack front is computed using the interaction integral. Subsequently, the maximum circumferential stress criterion and Paris’ law are employed to determine the current crack propagation direction and rate. Finally, finite element analysis platforms, ABAQUS and FRANC3D, are utilized to analyze the variation in crack propagation laws as influenced by the SIF distribution. The findings indicate surface cracks predominantly exhibit mixed-mode propagation comprising opening, sliding, and tearing types, while deeper cracks tend to show combined opening and sliding types. As the leading edge of the crack approaches the rail’s surface, the equivalent SIF increases, thereby facilitating crack propagation and transforming the semicircular crack into an elliptical configuration, with the major axis oriented in the depth direction.