Air quality inside metro systems is a critical issue for commuters and metro employees because of the considerable time they spend traveling or working in such systems. This study measured the PM10, PM2.5, particle number (PN), and CO2 levels and particle mass size distributions inside operating trains to reveal the factors influencing the concentrations and size distributions of particulate matter (PM) inside metro train carriages traveling in different environments. Measurement results demonstrated that the CO2 level was associated with ridership and that the PM levels and particle mass size distributions were remarkably affected by the surrounding conditions of the trains, such as their journey in underground tunnels, on ground-level tracks, and in different travel directions. Furthermore, the particle mass size distributions inside the metro train carriages exhibited a clear triple-mode pattern. The particle mass size distribution patterns between the ground-level and underground routes were considerably different due to their different potential sources of particles.