Sector coupling between the building and transportation sector can reduce decarbonization costs by boosting the energy system's overall performance, adequacy, flexibility, reliability and economy while leveraging renewable energy. This project recommends that the net-positive energy metro railway system is a large-scale versatile system capable of integrating all proven technologies to resolve many energy-related issues. Low consumption, high-efficiency metro station building and battery-powered metro trains with B2V, G2V, V2B and swappable battery techniques are designed, modelled, parametrically simulated and analyzed with various control strategies to achieve a net-positive energy balance based on various techno-enviro-economic indicators. Six different metro railway systems have been comprehensively developed and comparatively studied to investigate the renewable energy integration and interaction among metro stations, metro trains and the utility grid while achieving optimum load matching and grid interaction. The economic and energy flexibility studies are carried out to quantitatively highlight the potential. When the net-positive energy principle is implemented in a traditional railway system (ref_case-2), the renewable energy output of 1907 MWh meet demand, resulting in annual net positivity of 15.36% with an OEF of 0.8 and OEM of 0.71, and reducing annual carbon emissions into −8.5 kg/m².a. The economic indicator shows that ref_case-2 and ref_case-3 are feasible only on Hong Kong FiT and not viable on the net-metering-based tariff. Eliminating the investment in electric catenary for the overhanging power line by following a battery-based metro train with B2V, G2V, V2B, and swappable battery technique significantly improves the overall performance and makes the system techno-enviro-economically feasibility on both the tariffs.