## 3 Verification
### Moving Model Test Verification  
To validate the accuracy of the numerical simulations in this study, an experimental setup mimicking a falling debris-tunnel-train scenario was established with a scale ratio of 1:20. The experimental apparatus was based on the high-speed railway moving model test platform at Central South University, as illustrated in Figure 6a. The chosen scale ratio adheres to the similarity criteria specified by the CEN standard [37].The experimental tunnel comprised two sections: transparent and opaque. The opaque portion, the primary segment of the tunnel, spanned a length of 10 m. The transparent segment, measuring two meters, enabled observation of the fall trajectory of the debris. The two tunnel sections were securely connected by sealant, ensuring effective sealing. The cross-sectional area of both tunnels was 0.25 m\({}^{2}\). The train model was proportionally downsized based on the common CRH380B three-group train in China at a 1:20 ratio, with negligible considerations for small components such as bogies, windows, and doors. The head and tail trains measured 1.29 m in length, while the middle train was 1.24 m long. The width and height were 0.16 m and 0.19 m, respectively. Additional details regarding moving model test platforms can be found in the literature [38; 39]. Following the methodology outlined in [PERSON] et al. [6] as a reference, a rectangular sheet for the falling debris was selected, featuring a length and width of 0.025 m and a thickness of 0.002 m. The chosen density for the debris was set at 2400 kg/m\({}^{3}\), closely approximating the density of concrete fragments. The experimental speed was maintained at 300 km/h. This experimental setup provided a physical validation framework for the numerical simulations conducted in the study. Figure 6: Comparison of (**a**) falling debris–tunnel–train moving model test device and (**b**) numerical simulation model. Before commencing the test, an infrared laser sensing switch was positioned at the tunnel's bottom, while a switch to secure the falling debris was installed at the top of the transparent tunnel. Throughout the experiment, the infrared laser was obstructed by the train as the head train's nose tip reached the drop position, causing the closure of the tunnel's top switch. This action prompted the release of the falling debris, allowing it to descend under the combined influence of gravity and the slipstream generated by the train. During the descent, a high-speed camera with a frame rate of 2000 frames per second (fps) recorded the trajectory of the falling debris. Simultaneously, to enhance the clarity of the captured images, an LED light was positioned beside the high-speed camera as a fill light. As depicted in Figure 6b, this research also establishes a CFD numerical model with tunnel length, cross-sectional area, and train speed parameters consistent with those of the experimental setup. The numerical simulation methodology aligns with that outlined in Section 2, utilizing a medium grid to discretize the computational domain. The entire computational domain comprises approximately 45 million grid elements. The data from the moving model tests were derived by averaging the results of ten individual tests. In Figure 7a,c, the falling trajectories of the debris obtained from both the moving model tests and CFD numerical simulation are presented, with the time axis defined relative to the moment when the debris initiates its descent (position 0). Additionally, Figure 7b illustrates the motion attitude of the debris at nine key moments (\(t_{1}{}^{\prime}\)-\(t_{9}{}^{\prime}\)) during the falling process. The outcomes reveal a notable agreement between the numerical simulation results and the experimental findings. The falling trajectory of the debris takes the form of an inverse parabola, moving in the opposite direction of the train. The maximum moving distances are \(-\)0.055 and \(-\)0.054, respectively, with a difference of 1.85%.