## 4 The Impact of Partial Oxygen Pressure and Temperature on Internal Pressure Fluctuations  
Under extreme tunnel conditions with special climates such as high cold and low oxygen, the internal and external environments of the train become more complex, with  
Figure 6: (**a**) Values of \(e_{rms}\) under different pressure loss correction coefficients; (**b**) comparison between measured and simulated internal pressure of the train passing through Tunnel 1. Figure 7: (**a**) Comparison of one-dimensional numerical simulation internal pressure with model calculation results; (**b**) Comparison of measured internal pressure and model calculation results as the train passes through Tunnel 2.  
multiple physical variables such as internal pressure, partial oxygen pressure, and temperature interacting with each other. This increases the demands on the comfort of the internal environment. However, the mechanisms by which these multiple physical variables affect passenger comfort under such harsh conditions have not yet been fully understood, making it difficult to establish a model for predicting their impact on passenger comfort. As a result, the effective control of these variables within the train environment remains challenging, and issues related to maintaining comfort in the carriage are likely to be exacerbated. Based on the established models of internal pressure, partial oxygen pressure, and temperature, it is evident that when the EMU train operates under extreme tunnel conditions, the variations in these physical quantities are interrelated and coupled. The influence of these multivariable factors on passenger comfort within the carriage is a process of mutual interaction. Investigating the coupling effects of partial oxygen pressure and temperature on internal pressure fluctuations and their impact on pressure comfort will provide a theoretical foundation for further research into the mechanisms of passenger comfort changes under the coupling of multiple physical variables. This will also aid in the development of models to predict changes in passenger comfort under varying physical conditions and contribute to the maintenance and regulation of overall comfort within the carriage. Given that the railway in the mountainous regions of western China is still under construction, which prevents experimental validation using measured tunnel pressure waves, this study aims to investigate the impact of oxygen supply and temperature on internal pressure fluctuations under extreme tunnel conditions. To accomplish this, extreme tunnel pressure waves generated by one-dimensional numerical simulations [20] will be utilized alongside the established multi-factor coupled internal pressure fluctuation model to analyze how partial oxygen pressure and temperature--two critical comfort variables--affect the internal pressure characteristics. The railway in the mountainous regions of western China generally follows an uphill gradient. The specific conditions selected for this study involve a train ascending at a speed of 200 km/h through an extreme tunnel that is 42.5 km long, with a gradient of 30% and an entrance elevation of 3600 m. The extreme tunnel pressure waves induced by the train-tunnel coupling, simulated using one-dimensional numerical methods, are shown in Figure 8. The equations for calculating temperature and pressure at different altitudes in the troposphere, as provided by the aerospace discipline, are given in Equations (44) and (45). \[T_{H}=T_{0}-aH \tag{44}\]  
\[P_{H}=P_{0}\bigg{(}1-\frac{H}{44330}\bigg{)}^{g/aR} \tag{45}\]  
where \(H\) is the altitude, \(a\) is the annual average temperature lapse rate, \(T_{H}\) is the atmospheric temperature at altitude \(H\), \(  
To assess the impact of extreme tunnel pressure waves on passenger comfort, many countries and the International Union of Railways (UIC) generally use the rate of internal pressure change as the evaluation standard for internal pressure comfort.