High-speed rail (HSR) is critical modern transport infrastructure, offering efficient intercity mobility. China, a global leader in HSR development, boasts the world's largest HSR network. While this development boosts economic activity and provides essential commuting services, it has also driven substantial material consumption and GHG emissions. Previous environmental assessments of railway systems have predominantly focused on reducing operational emissions, overlooking the significant upstream impacts of material extraction, processing, and construction. The trade-off between pre-use (embodied) and in-use emissions is crucial; faster HSR speeds often require more energy-intensive materials. Understanding the full life-cycle material metabolism and GHG emissions of HSR is therefore essential, especially considering China's ambitious goal of carbon peaking before 2030 and carbon neutrality before 2060. China plans to expand its HSR network significantly by 2035, necessitating substantial material resources and raising sustainability concerns. While construction technologies have advanced, their environmental implications, particularly embodied emissions within the material metabolism, remain insufficiently studied. Material metabolism analysis offers a systematic framework for understanding material flows (inflows and outflows), stocks (materials within the system), and development patterns of built environments. Such studies have been applied to buildings and specific materials like steel and cement, but comprehensive analysis at the national HSR level is lacking. Existing studies often focus on individual lines or limit analysis to stock estimations. This research addresses these gaps by analyzing the spatiotemporal development patterns, material metabolism characteristics, and environmental impacts (GHG emissions) of China's HSR system. The study addresses three key research questions: (1) What are the development patterns and material metabolism characteristics in China's HSR? (2) What are the environmental impacts of GHG emissions in Chinese HSR development? (3) How does HSR compare environmentally to road and aviation passenger transport? The analysis employs bottom-up material flow and stock modeling with geographical data to characterize the spatiotemporal development patterns, material stock diversities, and contributions of technological advancements to material consumption reduction. The study also quantifies the carbon replacement value (CRV) and operational GHG emissions of HSR, comparing them to road and aviation to highlight HSR's emission mitigation potential within the broader context of Chinese passenger transport infrastructure.

The existing literature highlights the importance of high-speed rail (HSR) as efficient intercity transport infrastructure, contributing to economic growth and improved mobility (Schmutzler, 2021; Zheng & Kahn, 2013). Studies have assessed the energy use and environmental emissions of HSR in China, focusing on operational emissions (Chang et al., 2019; Lin et al., 2021). Life cycle assessments (LCAs) of HSR in China have also been conducted (Yue et al., 2015), but often lack comprehensive coverage of material metabolism. The IEA has examined the overall energy use and emissions of rail transport globally (IEA, 2019), showcasing its potential for reduced energy demand compared to road and air transport. However, the trade-off between reduced operational emissions and increased embodied emissions due to material use in HSR construction has been a less explored area (Venkatraj et al., 2020; Pauliuk et al., 2021). Research on material metabolism in built environments has focused on buildings (Camarasa et al., 2022; Göswein et al., 2019) and specific materials (Pauliuk et al., 2013; Cao et al., 2017), but nationwide HSR-specific analysis remains scarce (Lee et al., 2020; Wang et al., 2016). While some efforts exist to estimate HSR material stocks (Wang et al., 2016), comprehensive material metabolism analysis at the national level is lacking. Studies examining the historical patterns and stock disparities among HSR lines are limited due to data scarcity (Lanau et al., 2019). The lack of comprehensive HSR material metabolism data hinders a complete understanding of future material consumption and strategic planning for sustainable resource use. Studies have touched upon the carbon replacement value (CRV) of HSR (Müller et al., 2013), but further investigation into its implications for national-level HSR planning is needed. Emerging big data technologies in urban science offer new possibilities for addressing these data limitations (Long, 2019; Haberl et al., 2021), though their application to HSR studies remains underdeveloped.

This study employs a bottom-up approach to model the material flows and stocks of China's HSR system from 2008 to 2035. Data were collected from multiple sources, including national statistical yearbooks, individual HSR construction company reports, on-site surveys, interviews with HSR constructors and contractors, official websites, and web-crawling techniques to access geographical distributions (primarily from Baidu Map). The HSR line database includes attributes such as line name, length, bridge-tunnel ratio, track length, tunnel length, bridge length, culvert length, speed, geographical location, and opening date (see Supplementary Data 1 and Supplementary Note 1.1). The HSR station database includes station name, volume, and class (Supplementary Data 2 and Supplementary Note 1.2). Socioeconomic data (population, GDP, land area) at both national and provincial levels were obtained from statistical yearbooks (Supplementary Note 1.3). Web-crawling data were validated using a rectification method (Li et al., 2022) to correct for map distortions. A labor-intensive compilation of the HSR Material Composition Indicators (MCIs) database was performed from bills of quantities covering over 50 HSR construction projects (see Supplementary Tables S2-S12, Figure S3). These materials were categorized into seven end-use components (bridge, subgrade, tunnel, track, station, culvert, and EPCS systems). A total of 19 construction materials were classified and further grouped into seven bulk material types for stock mass calculations. The bottom-up stock accounting method (Equation 1) calculates material stocks (MS) by summing the product of the physical size (PS) of each HSR component and its material composition intensity (MCI): MS = Σ(PS × MCI). Material inflows (resource demands) and outflows (construction waste) were determined using mass balance principles (Equation 2), considering differences in material stocks between consecutive years, maintenance flows, and material loss (primarily from temporary facilities and formwork). Spatial analysis was conducted at the provincial level (excluding Macau and Taiwan) using ArcGIS 10.2, integrating stock estimations with the geographical inventories of HSR lines and stations. Gini coefficients were calculated to quantify regional inequality in HSR stock distributions. The carbon replacement value (CRV), representing the GHG emissions from using current technologies and materials for HSR construction, was estimated using Equation 3: CRV = (EF × MS), where EF is the emission factor of each material, obtained from the Chinese Life Cycle Database (CLCD) and literature (Supplementary Table S18). Operational GHG emissions for road, aviation, and HSR transport were estimated using Equation 4: OE = Σ(OEF × PT), where OEF is the operational emission factor and PT is the passenger turnover volume (Supplementary Method 2). Uncertainties in HSR stock and emission estimations were addressed using Monte Carlo simulation (10,000 runs) with a 95% confidence interval for parameters (Supplementary Figs. S24-S27). Sensitivity analysis identified key sources of uncertainty. Data on road and aviation were collected using similar methods (Supplementary Tables S13-S17).

This study reveals several key findings regarding the material metabolism and GHG emissions of China's HSR system: **Development Patterns and Material Stocks:** * The HSR network expanded rapidly from 1039 km in 2008 to 38,914 km in 2020, exceeding 60% of the global length. The cumulative material stock increased from 0.13 Gt in 2008 to 3.65 Gt in 2020, surpassing global subway stocks in 2020 and global in-use plastic stocks in 2014. The projected stock is 4.9 Gt by 2035. * Nonmetallic minerals (gravel, sand, cement) constituted 88% of total HSR stocks in 2020. Bridges and subgrades represent the largest end-use components, with material stock composition varying across different HSR lines depending on geographical characteristics (bridge-tunnel ratio, terrain). The Beijing-Shanghai HSR line, for instance, shows a high proportion of bridge stocks, while the Hainan East line is dominated by subgrade stocks. Mountainous regions display higher tunnel stock proportions. * Higher HSR speeds (350 km/h) initially increased material consumption, particularly for steel rail, subgrade, and tunnels compared to slower lines (200 km/h). However, advancements in construction technologies, such as long-span box girders and ballastless tracks, offset some of the increased material demand, resulting in a decreasing HSR material density (from 110 t/m in 2008 to 92 t/m in 2020). The adoption of ballastless tracks reduced aggregate consumption. **Spatial Distribution of Stocks:** * The spatial distribution of HSR material stocks across Chinese provinces has become more balanced since 2010, largely due to the government's Great Western Development initiative. However, disparities remain, with the Southeast possessing significantly more HSR stocks than the Northwest in 2020. * Variations in per-area and per-capita stocks highlight the uneven distribution of HSR infrastructure across provinces. Shanghai exhibits a high per-area stock, while western regions show considerably lower values. Disparities in per-capita stocks are observed, potentially due to factors like population density and the extent of HSR development. **Material Flows and Waste Generation:** * The material flow analysis maps the significant demand for aggregates (gravel and sand), particularly in concrete engineering. The study quantifies the large volume of these materials used in HSR construction, highlighting the potential for aggregate shortages and related environmental concerns like coastline erosion and ecosystem degradation. * Only a small amount of waste (26.3 Mt) was generated by 2020, mainly from temporary construction facilities. Low recycling rates were observed (e.g., 0.68 Mt of recycled steel from formwork), with the majority of HSR waste landfilled. **Greenhouse Gas Emissions:** * The CRV of transport infrastructure in 2020 reached 2.5 Gt, comparable to the 2014 CO2 emissions of China's cement industry. While HSR showed a substantial CRV, the difference compared to road transport is narrowing over time. Cement and steel account for the largest portion of HSR CRV. * Operational GHG emissions from intercity passenger transport totaled 143 Mt in 2020. Aviation contributed the most operational emissions, followed by road and HSR transport. The COVID-19 pandemic impacted passenger volumes and thus emissions across all three transport modes. * Comparing environmental impacts per passenger-kilometer (pkm), HSR demonstrates significantly lower material requirements and CRV compared to road transport, while aviation shows the lowest per-passenger material demands and CRV. Aviation exhibited higher operational emissions per pkm than road and HSR transport.

The findings demonstrate that while China's HSR expansion has significantly increased material consumption and GHG emissions, technological advancements and infrastructure planning have mitigated some of these impacts. The narrowing spatial distribution gaps across provinces indicate progress towards more equitable infrastructure access. However, disparities remain, requiring targeted policies to further balance HSR development regionally. The decreasing material density in HSR construction highlights the efficacy of technological innovations, such as long-span box girders and ballastless tracks. The relatively low waste generation from HSR operation suggests opportunities to further enhance resource efficiency and waste management through circular economy principles. The analysis highlights that while HSR offers significant environmental benefits over road transport, the embodied emissions associated with the construction of HSR infrastructure and its material content remain substantial. The substantial proportion of cement and steel in HSR construction underscores the importance of developing and deploying low-carbon technologies in these sectors, such as hydrogen-based steel production and carbon capture and storage in cement production. Aggregate shortages represent a critical challenge, necessitating the exploration of substitute materials and optimized construction practices. Comparing environmental impacts across transport modes (HSR, road, and aviation) demonstrates HSR’s significant environmental advantages in terms of material consumption and CRV per passenger-kilometer, though aviation remains most efficient in terms of operational emissions. Future work should incorporate detailed life cycle assessments, consider the whole transport chain, and examine the interaction of other economic sectors, urban forms, and land use patterns. Further research is necessary to fully integrate the findings into urban climate strategies and planning approaches for emission reduction.

This study provides a comprehensive assessment of the material metabolism and GHG emissions of China's rapidly expanding HSR network. Key findings reveal the significant material consumption, the role of technological advancements in mitigating resource demands, the ongoing spatial inequalities in infrastructure distribution, and the environmental benefits of HSR compared to road and air transport. The study underscores the urgent need for low-carbon technologies in the steel and cement industries, along with the exploration of aggregate substitute materials and optimized construction practices. Future research directions should focus on more granular life cycle assessments, incorporating wider economic and social contexts, and enhancing waste management strategies for a truly sustainable HSR future.

While this study presents the most comprehensive national-level analysis of China's HSR material metabolism and GHG emissions to date, several limitations exist. Data limitations may lead to uncertainties in material composition indicators (MCIs) and emission factors. Spatial resolution could be improved for more detailed analyses of HSR components. The study's focus on China limits the direct generalizability of findings to other countries. Future research should aim to refine data collection, expand spatial and temporal resolution, and account for wider socioeconomic factors to enhance the robustness and generalizability of the results.