• Perception of crowding is conditioned by several interacting variables. • Number of passengers, perceived noise and smell explain 46.5% of perception of crowding while riding subway. • Perceived noise is more important in the perception of crowding than passenger density. • Measured noise level is unrelated to perceived noise level or passenger density. Public transport is being promoted worldwide to help resolve the environmental and congestion problems besetting cities. Travel comfort is increasingly seen as crucial to effecting the switch from private motorized modes to public transit. The purpose of this study is to measure the physical and human factors impacting travel comfort on mass urban rail transport. Randomly selected passengers (n = 368) commuting in subway cars at 14 levels of in-vehicle passenger density were intercepted to evaluate their satisfaction with the following factors related to comfort: crowding, noise, smell, air quality, temperature, illumination, vibration and safety from crime. Satisfaction with regard to crowding was negatively and linearly related to the number of passengers in the subway car. In the multiple regression model, however, perceived noise intensity accounted for more than the actual number of individuals in the car in the evaluation of crowding, with smell as a second significant contributor. Other significant factors in travel comfort included air quality, temperature, and fear of crime. Although it is found that objectively measured passenger crowding explains a major part of perceived travel comfort, other factors associated with the ambient environment are at least as important. Corrective measures to improve travel comfort could address the ambient environmental factors in the medium term, while the structural issue of over-crowding needs to be addressed in the longer term.