As global warming intensifies, reducing carbon emissions has become a consensus among the international community. The China-Europe Railway Express, as a crucial component of the "Belt and Road" initiative, contributes to the development of China-Europe trade; however, its impact on mitigating carbon emissions has not been fully studied. Existing research primarily focuses on the economic benefits of the China-Europe Railway Express, with empirical analyses in the field of carbon reduction being relatively scarce. This study aims to uncover the key 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 the absence of the China-Europe Railway Express and compares these predictions with the actual scenario where the China-Europe Railway Express exists. 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. The findings reveal that the establishment of the China-Europe Railway Express has significantly altered the share of various transportation modes in China-Europe trade. Specifically, carbon emissions were reduced by approximately 1.18 million tons in trade from China to the EU and by about 1.69 million tons in trade from the EU to China, 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, in the long run, the strategic significance of the China-Europe Railway Express in optimizing transportation structure and mitigating carbon emissions remains irreplaceable. 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.