Green and low-carbon development is the theme of today's urban construction. Rail transit is an important transport infrastructure in cities, which affects urban transport carbon emissions. The impact of urban rail transport on urban carbon emissions was empirically examined. The results show that the construction of urban rail transport has a significant effect on the reduction of urban carbon emissions, and there is heterogeneity in the effect of urban scale. Urban rail transport has produced the “transport substitution” effect, increased new investment opportunities, increased green financial inputs, promoted green technological innovation, and thus reduced urban carbon emissions.