Growing global concerns about resource depletion and environmental degradation have highlighted the potential role of high-speed railways (HSR) in promoting sustainable development. China, the world's largest developing country and a major energy consumer, faces significant environmental challenges due to its reliance on fossil fuels and high emission levels. The transport sector is a major contributor to global carbon emissions, making the environmental impact of HSR, a rapidly expanding mode of transport in China, a critical area of study. This paper addresses the following research questions: Does HSR operation significantly contribute to nationwide energy savings and emission reductions? How does HSR promote ecological environmental sustainability? Existing research on the topic shows mixed results, with some studies suggesting positive environmental effects while others highlight potential negative impacts. There is a lack of comprehensive analysis incorporating both macro-city and micro-enterprise data, along with insufficient understanding of the spatial spillover effects of HSR operations. This research aims to fill these gaps.

The literature review traces the development of China's high-speed rail network from its inception to its current status as the world's largest and most advanced system. Early research focused on the history and planning of the network. Later studies examined the economic impacts of HSR, including its effects on tourism, regional development and economic growth, both within connected cities and in those not directly linked to the network. Recent work has explored the relationship between HSR and technological progress, industrial agglomeration, and environmental protection. While some studies highlight the positive environmental effects of HSR, such as reducing emissions from other transportation modes, others suggest that HSR can exacerbate carbon emissions due to increased economic activity and industrial development. However, much of the existing research lacks a comprehensive analysis that uses both macro-level data and micro-level data from individual industrial enterprises. The research also often focuses on specific regions along high-speed rail lines, overlooking the broader spatial spillover effects that can occur across wider areas.

This study employs a quasi-natural experiment approach using a difference-in-differences (DID) method, specifically a staggered DID model, to account for the varying timing of HSR openings across different cities. The analysis utilizes panel data from 285 prefecture-level cities in China from 2003 to 2020 and micro-enterprise data from 2003 to 2012. The dependent variables are measures of environmental sustainability, including fossil fuel consumption (coal and oil) and carbon emissions (total and per capita). The key explanatory variable is a dummy variable indicating the presence and timing of HSR operation in each city. A difference-in-differences-in-differences (DDD) model is also employed to investigate the impact of HSR on different types of industries. Spatial DID (SDID) models are used to capture the spatial spillover and conduction effects of HSR. Control variables are included to account for various economic and environmental factors at both macro and micro levels, including industrial structure, infrastructure construction, high-end talent, fiscal autonomy, population density, FDI, enterprise size, age, and financing capacity. Robustness checks are conducted using propensity-score matching (PSM-DID), placebo tests, and instrumental variable analysis to address potential endogeneity concerns. Heterogeneity analysis explores variations in the impact of HSR based on city type, resource endowment, and population size. Urban carbon emissions are calculated using an equation based on the carbon emissions of natural gas, liquefied petroleum gas, and electricity consumption.

The key findings of the study unequivocally demonstrate a positive environmental impact of HSR operations in China. At the micro-enterprise level, the introduction of HSR is associated with significant reductions in traditional fossil fuel consumption, particularly coal. This effect is more pronounced in high-carbon industries, capital-intensive industries, human-intensive industries, and state-owned enterprises. A significant distance attenuation effect is observed; that is, the reduction in fossil fuel consumption diminishes as the distance from HSR stations increases. The study finds that HSR in pivot cities leads to a greater reduction in fossil fuel consumption. At the macro-city level, HSR operation is associated with significant reductions in both total and per capita carbon emissions. This effect is stronger in third-tier cities, non-megacities, and mature resource-based cities. The study identifies significant spatial spillover and conduction effects, indicating that the environmental benefits of HSR extend beyond the cities directly served by the railway lines. The analysis of mechanism pathways shows that HSR fosters labor productivity gains, promotes industrial structure upgrading, accelerates elements flow, and stimulates technological innovation, particularly in green technologies, all of which contribute to environmental sustainability. The robustness of these findings is reinforced by a series of sensitivity analyses and robustness checks, including the propensity-score matching, placebo tests, and instrumental variable analysis. The study also addresses selection bias and endogeneity concerns in its analysis and provides a clear analysis of various urban heterogeneity effects.

The findings directly address the research questions by showing that HSR operation has a positive and significant impact on environmental sustainability in China, reducing fossil fuel consumption and carbon emissions. The positive effect is not confined to cities directly connected to the railway network, but also extends to neighboring cities through spatial spillover and conduction effects. The mechanism analysis reveals the pathways through which HSR achieves these positive environmental outcomes, highlighting the interplay of economic and environmental factors. These findings are particularly significant given China's large energy consumption and significant carbon emissions. The results contribute to the existing literature by providing comprehensive evidence based on both macro and micro data, addressing limitations of previous research and providing a deeper understanding of the complex relationship between HSR, economic development, and environmental sustainability. This research complements the growing body of work on the environmental impact of transportation systems, providing much-needed empirical evidence from a large-scale and rapidly developing economy.

This study provides robust evidence supporting the environmental benefits of HSR in China. The findings show that HSR significantly reduces fossil fuel consumption and carbon emissions, and these impacts are achieved through multiple mechanisms, including improved labor productivity, industrial structure upgrading, accelerated element flows, and technological innovation. The research highlights the importance of considering the spatial spillover effects of HSR when assessing its overall environmental impact. Future research could explore the long-term sustainability of these effects, investigate the impacts of different HSR technologies, examine the role of policy design in maximizing the environmental benefits of HSR, and conduct similar studies in other countries with varying levels of economic development and transportation infrastructure.

The study acknowledges some limitations. The carbon emission proxies used, while aligning with China's dual-carbon policy, might not fully capture the nuanced aspects of urban emissions. The analysis focuses primarily on China's context, and the generalizability of the findings to other countries with different economic, political, and environmental circumstances might be limited. Future research could refine carbon emission measurement and extend the analysis to other national contexts.