This study aims to determine the correlation between CO2 concentrations, CO concentration, relative humidity, and temperature to the number of train passengers. Even though the number of passengers is growing, some reports assert that the quality of the indoor air of Malaysian passenger trains is controversial, particularly during peak hours. The survey included 28 major stations and was carried out over 2 h on weekdays at peak and non-peak hour travel times. The direct-reading instrumentations were in the middle of the train compartment and used for temperature, humidity, air movement, carbon monoxide (CO), and carbon dioxide (CO2). The number of passengers in and out of the train was also counted manually. The finding indicates that the concentration of CO2 (up to 1380 ppm) during peak hours exceeded the standard limits (1000 ppm) set by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), and Department of Occupational Safety and Health (DOSH) Malaysia. A positive correlation exists between the number of passengers and CO2 concentration. However, the other parameters such as relative humidity, temperature, air movement, and CO concentration have weak correlations to the number of passengers. This study can provide initial data to the train management team on the requirement to improve indoor air quality by applying suitable control measures such as a proper ventilation system.