The diffusion of pollutants from diesel locomotives in high‐altitude railway tunnels

With the continuous expansion of high‐altitude railway construction, pollutants emitted by diesel locomotives operating in long railway tunnels have become a major concern for tunnel air quality and passenger health. Therefore, a CFD‐based coupled tunnel–train–carriage model was established to investigate the dispersion characteristics of CO and NOx. The effects of fresh‐air ratio, train speed, and longitudinal ventilation velocity on pollutant distribution were systematically analyzed. The results indicate that NOx is the dominant pollutant affecting passenger exposure in high‐altitude railway tunnels. Pollutant concentrations within the tunnel were closely associated with train position, whereas in‐car pollutant concentrations continuously increased during train operation. Increasing the fresh‐air ratio from 20% to 50% increased the in‐car NOx concentration by approximately 104%. Even at a 20% fresh‐air ratio, the in‐car NOx concentration remained more than 100 times higher than the guideline value of 0.25 mg/m 3 . Increasing train speed significantly reduced pollutant concentrations in both the tunnel and passenger compartments. Dimensionless analysis revealed similar attenuation patterns for CO and NOx, despite their different concentrations. However, even at 110 km/h, the in‐car NOx concentration still exceeded the guideline value. Longitudinal ventilation reduced pollutant residence time within the tunnel but increased pollutant concentrations near roof‐mounted fresh‐air intakes, resulting in higher in‐car exposure levels. These findings provide a theoretical basis for tunnel ventilation design, train operation management, and air quality control in high‐altitude railway systems.