Does the development of China?s high-speed rail improve the total-factor carbon productivity of cities?

China's high-speed rail (HSR) has developed expeditiously since the beginning of the 21st cen-tury, exerting significant influences on many aspects, such as economic development and resi-dents' travel mode. Improving carbon productivity is one of the necessary measures to realize China's carbon neutrality goal, considering economic growth. However, previous researches have not dealt with the exact impact of HSR opening on carbon emission performance. This study seeks to fill this gap. Through the difference-in-differences (DID) model, we discover that the opening of HSR significantly improves the city's total-factor carbon productivity in China. In addition, the influencing mechanism and heterogeneity of the impact are discussed, and the polarization effect is also analyzed. Overall, this study strengthens the idea that HSR construction has positive environmental externalities. The insights gained from this study may be of assistance to accu-rately formulate policies related to HSR planning and construction in the future.