China's urban rail transit carbon peak: when and how – a dynamic life cycle assessment

China's rapidly expanding urban rail transit (URT) system faces significant environmental challenges, particularly regarding the inflexibility and structural nature of passengers’ urban transportation demands. It is a key issue to assess the carbon peak timeline and related technological roadmap for carbon peaking in China’s URT sector. This study conducts a comprehensive life cycle assessment to evaluate carbon dioxide emissions across different URT life cycle phases. Through input factor identification and scenario modelling, the authors investigate critical factors affecting life cycle emissions and project emission data from 2019 to 2035. The analysis results suggest that there was an 11% reduction in emissions in 2020, which is attributed to the shifts in transportation demand induced by COVID-19. Among various development scenarios, only the ambitious scenario, which involves the adoption of decarbonisation technologies and robust policy enforcement, achieves carbon peaking by 2030. To support emission management, the authors propose a multi-level framework that combines upstream data integration with goal decomposition methodology. This framework is based on macro-level sectoral assessments and the development of technology-specific roadmaps, thereby supporting the sustainability of the URT system in line with China's national climate objectives while maintaining transportation efficiency.