Experimental Study on Stiffness Degradation of CRTS III Ballastless Track–Girder System Under Train Load

A physical model of the China Railway Track System III (CRTS III) ballastless track was cast at a 1:4 scale, adhering to the principle of mid-span concrete stress equivalence. A multistage amplitude fatigue test, composed of 9 million cycles, was then conducted on the CRTS III–girder system. The results indicated that, first, no cracking or failure occurred in the CRTS III–girder during the experiment. The static deflection at mid-span and the concrete strain at the bottom of the box girder web increased significantly. However, throughout the experiment, the neutral axis of the girder remained unchanged. Second, the static stiffness of the CRTS III–girder and the static stiffness coefficient of the box girder concrete initially experienced a rapid decline, followed by a period of slow fluctuation, and then a subsequent rapid decline. Third, the degradation of dynamic stiffness in the CRTS III–girder mirrored that of the static stiffness. The changes in dynamic stiffness across the three stages were 14.79, 4.85, and 22.87 kN/mm, respectively. Consequently, the dynamic stiffness of the CRTS III–girder decreased to 93.33%, 89.08%, and 78.29% of its initial value across the three stages. Finally, the experimental results closely aligned with the theoretical predictions, demonstrating strong consistency between theory and experiment.