In this study, we monitored the carbon dioxide (CO2) concentration in a high-speed train passengercabin by the in-situ non-dispersive infra-red (NDIR) method in order to investigate the effects of variousfactors, such as number of passengers and the presence of tunnels on the CO2 concentration levels.We found that the CO2 concentration was strongly related with the number of passengers andcorrelated with the presence of tunnel regions. The CO2 concentration increased with increasingnumber of passengers and increasing tunnel residence time. This phenomenon could be observed inthe most of high-speed trains in the world, since the flap of ventilation equipment is closed to avoid theear-discomfort of passengers by pressure changes passing the tunnel section. As a result of thistemporary stop in the supply of fresh air, the concentration of CO2 in the passenger cabin increasesdrastically. We surveyed the tunnel information of the high-speed train in Korea, and divided the wholeline into several sections in order to correlate it with the CO2 levels of the passenger cabin. The carbondioxide levels of the passenger cabin often observed to exceed 2,500 ppm in tunnel section. Somealternative ways to keep the carbon dioxide levels lower in the tunnel section were suggested.