Abstract: Internal combustion locomotives are widely used in non-electrified railways in remote, high-altitude mountainous areas. However, during their operation, a high-temperature buoyant jet emanating from the roof can lead to localized overheating of the roof surface, potentially undermining the train's operational safety. This paper conducted numerical simulations leveraging numerical methods validated through actual vehicle tests of five trains (including two internal combustion locomotives) operating in high-altitude regions to explore the diffusion characteristics of high-temperature buoyant jets under varying operational velocities and jet incidence angles. The results indicate that as the running speed increases, the high-temperature airflow moves closer to the train surface. Particularly, when the internal combustion locomotive operates at its peak speed of 160 km/h, the roof surface experiences the largest high-temperature region. Conversely, increasing the jet incidence angle can effectively mitigate the impact of the incoming flow on the high-temperature buoyant jet so that the jet trajectory is elevated farther away from the roof and the roof surface temperature decreases. Specifically, when the incidence angle of the high-temperature buoyant jet attains 45°, the maximum temperature of the roof decreases by a significant 51.6%. The findings of this study offer crucial insights for mitigating the risk of damage to internal combustion locomotive roof equipment on high-altitude railways.