Public transit is viewed as a potential means to mitigate traffic congestion and its resulting air pollution and health consequences. A recent wave of studies has emerged examining whether this public investment can achieve its goals. Employing the difference in difference method, this paper examines the medium-term effect of the opening of a completely new subway line on local air pollution in Changsha. Our findings show that carbon monoxide pollution, one key type of automobile pollution in areas close to the subway line experienced a greater reduction relative to areas further away from the stations in the first year after the subway line opened. However, there is no evidence that the opening of the subway line affected particulate matter or ozone pollution. Heterogeneity effects support that air pollution reduction resulted from substituting subway use for road use.