Carbon Emissions of Railways: An Overview

Reducing greenhouse gas (GHG) emissions from transport is critical to global sustainable development efforts. Large-scale railways have the potential to reduce GHG emissions significantly throughout their life cycle, thus considered an effective solution. However, a comprehensive evaluation on the research landscape of railway carbon emissions (RCEs) is still missing in the current body of knowledge. This study conducted an extensive review of 15,881 articles from the Web of Science and Scopus databases, selecting 197 publications from 1999 to September 2024 for analysis. Using CiteSpace for keyword co-occurrence and clustering analysis, preliminary research themes were identified. This was followed by a detailed literature review using content analysis to examine research trends and themes deeply. The findings indicate a growing interest in RCEs research, especially after 2015, with most coming from China (82 articles) and the United States (23 articles). Seven research themes for RCEs were identified, with measurement methods (49 articles), impact on carbon emissions (39 articles), and mitigation measures (69 articles) as leading ones. Various measurement methods, including life cycle assessment, emission coefficient method, input-output model, and field monitoring method, are utilized, however, challenges related to activity data and emission factors hinder measurement accuracy. Railways influence carbon emissions through transport substitution, industrial structure upgrading, and technological innovation, with spatial spillovers and regional heterogeneity, however, inconsistencies arise regarding the directionality of this impact, whether positive or negative, and the underlying mechanism remains inadequately illustrated. Mitigation measures include policy level, technological innovation, and project management, nonetheless, each specific measure requires further empirical investigation and should be integrated to form a comprehensive governance. Additionally, more attention should be given to research in developing countries. This review provides the first comprehensive analysis of RCEs, highlighting key areas for future research and policy development, and offering valuable insights for researchers and practitioners.