A method for calculating the required amount of air for gas ventilation of the internal combustion engine of railway tunnels has been developed. This method takes into account such provisions as the direction of air movement to the direction of train movement, train speed and the number of trains simultaneously in the tunnel. Cases such as a train standing motionless with the engine running, train movement towards and against the direction of airflow, and cases of multiple trains in the tunnel are analyzed. For all variants, calculation equations of the required air quantity and recommended train speed that can ensure the concentration of pollutants in the tunnel below the MAC (maximum permissible concentration) are proposed.