## 5 Conclusions  
This study explores the issues of internal pressure fluctuations and the resulting passenger discomfort, caused by the 'train-tunnel' coupled aerodynamic effects as trains pass through tunnels under extreme conditions of high altitude, steep gradients, and long tunnels in the mountainous regions of western China. A multi-factor coupled internal pressure fluctuation model was developed specifically for these extreme tunnel conditions. The research began by analyzing the characteristics of external tunnel pressure waves to identify the factors influencing internal pressure fluctuations. Subsequently, the study investigated the mechanisms and developed models of internal pressure fluctuations under both single-factor and multi-factor coupling scenarios. The proposed model was then refined and validated using simulation data alongside actual railway line measurements. Finally, the study separately examined the effects of two key comfort variables--partial oxygen pressure and temperature--on the interaction of pressure fluctuations and their impact on pressure comfort. This research provides essential theoretical support for future exploration of passenger comfort dynamics under the interaction of multiple physical factors and for advancing technologies designed to maintain and regulate comfort in complex environments. The main conclusions are as follows:  
1. The factors influencing internal pressure fluctuations under the disturbance of extreme tunnel pressure waves were identified, including ventilation ducts, airtight gaps, body deformation, the oxygen supply system, and temperature. The mechanisms of internal pressure fluctuations under the impact of individual factors were clarified, and corresponding single-factor pressure fluctuation models were established. The  
Figure 14: Maximum internal pressure variation under different temperature standards. Figure 13: Internal pressure peaks under different temperature standards. (**a**) Maximum positive pressure; (**b**) Maximum negative pressure; (**c**) Peak-to-peak value. coupling mechanisms between the various transmission pathways were analyzed, and a multi-factor coupled internal pressure fluctuation model was developed based on the ideal gas polytropic process assumption and the mass conservation theorem. The pressure loss coefficient of the established model was adjusted using actual railway line measurement data, resulting in a corrected regression system fit standard deviation of 20.82 Pa, which is less than 5% of the pressure wave amplitude. Finally, the feasibility and validity of the model were confirmed through comparisons with measured data and one-dimensional simulation data. 2. Based on the revised internal pressure fluctuation model, an analysis was conducted to calculate the coupling effect and impact of changes in partial oxygen pressure--a key comfort-related physical variable--on internal pressure fluctuations under extreme tunnel conditions. The results indicate that, with a fixed valve opening, higher oxygen supply standards exert a greater influence on internal pressure fluctuations, with an impact that is more significant than that of internal temperature, accounting for up to 18.11% of the pressure fluctuation amplitude. As the valve opening decreases, the influence of oxygen supply standards on internal pressure fluctuations increases, with an overall impact increase of approximately 12.23%, suggesting a certain coupling effect between the oxygen supply system and the air ducts. 3. An analysis of the coupling effect and impact of temperature, as a comfort-related variable, on internal pressure fluctuations reveals that, with a fixed valve opening, higher temperatures lead to a greater influence on internal pressure fluctuations, with the maximum impact accounting for approximately 5.34% of the pressure fluctuation amplitude. Temperature has a mitigating effect on internal pressure as the EMU train ascends through extreme tunnel conditions.