Abstract: With increasing global warming, reducing carbon emissions has become a common goal for the international community. China–Europe Railway Express, as a flagship project of the Belt and Road Initiative, contributes to the development of China–Europe trade; however, its impact on mitigating carbon emissions has not been extensively studied. Existing research has primarily focused on the economic benefits of the China–Europe Railway Express and empirical analyses of its impact on carbon reduction are relatively scarce. Consequently, this study aims to uncover the role of the China–Europe Railway Express in carbon emission mitigation. It employs the autoregressive integrated moving average (ARIMA) model to simulate and predict changes in the share of railway transportation in China–Europe trade in the absence of the China–Europe Railway Express and compares these predictions with actual scenarios in which the China–Europe Railway Express is operational. Additionally, the study adopts a bottom-up approach using the carbon emission calculation method based on the total cargo turnover to compute the carbon emissions of the China–Europe Railway Express. Results reveal that the establishment of the China–Europe Railway Express has notably altered the share of various transportation modes used in the China–Europe trade. Specifically, while trading from China to the China–Europe and vice versa, carbon emissions were reduced by approximately 1.18 and 1.69 million tons, respectively, resulting in a total reduction of approximately 2.87 million tons of carbon emissions in both directions. However, the Russia–Ukraine conflict in 2022 negatively impacted the transport and carbon reduction effects of the China–Europe Railway Express. Despite this setback, in the long run, the strategic importance of the China–Europe Railway Express in optimizing the transportation structure and mitigating carbon emissions remains irreplaceable. Thus, this study contributes to a deeper understanding of the carbon reduction effects of the China–Europe Railway Express and provides a reference for decision-making in promoting the green development of the Belt and Road Initiative.