High-speed railways reduces carbon emissions: mediating effects of green innovation and the resilience of environmental investment

High-speed rails (HSRs) are a sustainable approach in many cities. Although some studies recognize that introducing HSR is negatively related to carbon emissions, explorations of the mechanisms underlying this relationship remain scarce. The main purpose of this study is to investigate whether introducing HSR reduces carbon emissions through increasing green innovation (GI) and the resilience of environmental investment (REI). Employing signal theory and fault tolerance theory, among others, we propose models that illuminate how inaugurating HSRs reduces carbon emissions through GI and REI. Our proposed central hypothesis is that GI and REI mediate carbon emission reduction by introducing HSR. In this study, we select Chinese cities from 20002021 as a sample and examine our hypotheses via a difference-in-differences (DID) model. The results show that introducing HSR reduces carbon emissions through GI and REI. On the basis of the current status of China's economic and social development, we further analyze several critical moderating effects, such as the digital economy, the urban‒rural gap, and whether it is a resource-based city. In 2023, China's Central Economic Work Conference (CEWC) explicitly proposed "developing the digital economy." We find that the digital economy strengthens the negative relationship between introducing HSRs and carbon emissions. The digital economy also strengthens the positive correlation between the introduction of HSR and GI. Additionally, this strengthens the positive correlation between HSRs and REI. In addition, the urban‒rural disparity is a manifestation of China's uneven development. We find that the urban‒rural disparity weakens the relationship between HSRs and carbon emissions. In addition, China's resource-based cities lag behind other cities. Our findings suggest that, in contrast to other cities, high-speed rails increase carbon emissions in resource-based cities.