Unlike the conventional automotive, modem railway trains are designed with non-openable windows; and a mechanical ventilation and air-conditioning (MVAC) system is installed in each train compartment for better indoor air quality as well as to provide a thermally comfortable environment. The ventilation rate is no doubt a critical element in the design of a MVAC system, especially in Hong Kong where the daily passenger load is extremely heavy. Earlier studies illustrated that carbon dioxide can be controlled at 1000 ppm by increasing the ventilation rate to 25.2 m3 h21; however, it will also lead to an increase in energy consumption. In this paper, the electrical energy consumption at various ventilation rates was studied, and the cost of maintaining a low carbon dioxide level was estimated. These provide solid information for the local railway companies to improve the air quality inside the train compartments.