Saudi Arabia's Green Leap: Unlocking Climate Potential of Haramain High Speed Rail Through Occupancy Optimization and Renewable Energy Adoption

HAFIZ Abdul WAJID^*

• Islamic University of Madinah, Medina, Saudi Arabia

This study presents the first comprehensive evaluation of the CO₂ mitigation potential of Haramain High-Speed Rail (HHR) system of Saudi Arabia using a dynamic scenario-based mathematical modeling approach. Focusing on the 450 km Makkah-Madinah corridor, the analysis examines four operational parameters including train energy efficiency (0.03-0.07 kWh/pkm), grid carbon intensity (0.65-0.10 kgCO₂/kWh), renewable energy (RE) adoption (0-100%), and occupancy rates (25-100%) while accounting for a 15% real-world system efficiency loss. Three key findings challenge prevailing norms in high-speed rail (HSR) planning. First, the HHR may emit up to 187 kT more CO₂ per passenger than diesel buses when occupancy falls below 25% and the system depends on a fossil-heavy grid. However, under full renewable energy adoption and high occupancy, it can save up to 285 kT CO₂ annually (11.93 kg per passenger). Second, occupancy rates exert an outsized influence on carbon performance. In multiple scenarios, ridership optimization yields greater emissions reductions than RE integration alone, particularly when grid decarbonization is partial. Third, the study identifies scenario-specific climate-positive thresholds: net CO₂ savings are achieved when occupancy exceeds 70-75% under the current grid mix, or 45-50% when RE adoption reaches 50%. The above thresholds highlight the nonlinear interplay between energy sourcing and ridership dynamics. The findings inform Vision 2030 and transport planning in fossil-dependent economies. By moving beyond static lifecycle assessments, the methodology integrates operational variability and passenger dynamics offering policymakers a practical toolkit to align clean energy investment with ridership incentives and forecast emissions under real-world conditions. Contributing to UN-SDGs 9, 11, and 13, this study also establishes a foundational reference for future research on decarbonization thresholds in HSR systems particularly across Middle Eastern and arid-region contexts.