Carbon dioxide (CO 2 ) emissions reduction has become an ever-growing concern in China and the government has proposed the goals of carbon peak and carbon neutrality. To better address this concern, this work pays particular attention to high-speed railway (HSR) operations and examines their spatiotemporal effects and potential mechanism on CO 2 emissions. Using a geographically and temporally weighted regression (GTWR) model to fit a balanced panel dataset from the period 2008 to 2018, we have the following main findings. First, the GTWR model performs better than the pooled panel regression model as it considers temporal and spatial variations in factors of CO 2 emissions simultaneously. Second, the temporally varying coefficients of HSR operations indicate their consistent contributions to emissions reduction, suggesting that the national development of HSRs can provide significant emissions reduction benefits. Third, as revealed by the spatially varying coefficients of HSR operations, most provinces can mitigate CO 2 emissions by promoting HSRs, particularly in Shanxi, Hebei, and Shaanxi, owing to the larger contributions of HSR operations to CO 2 emissions reduction. Finally, the contributions of HSR operations to emissions reduction can be transmitted through the mechanism of technological progress. These findings can offer valuable insights into cross-collaborative and province-specific policymaking for mitigating CO 2 emissions.