Is Metro-based logistics always greener? A carbon emission break-even analysis under the trend of electrification and multi-dimensional spatiotemporal constraints

Metro-based Underground Logistics Systems (M-ULS), which leverage spare capacity in urban rail transit networks, have been widely regarded as a promising solution for alleviating urban congestion and reducing carbon emissions. However, as battery electric trucks (ET) rapidly proliferate and regional electricity grids progressively decarbonize, the traditional binary comparison of “metro versus diesel trucks” is no longer a sufficient basis for claiming M-ULS environmental superiority. Two questions remain unresolved: whether M-ULS retains meaningful emission advantages over electric trucks under realistic urban operating conditions, and how road congestion, ambient temperature, and metro stop-start dynamics jointly shape this comparison. This study develops a refined carbon accounting model that compares M-ULS with both electric trucks and conventional diesel trucks (DT) within a unified framework. The model incorporates a BPR (Bureau of Public Roads) function-based congestion coefficient and powertrain-specific temperature correction factors. These are applied symmetrically to all road-based segments, including M-ULS drayage, under a consistent well-to-wheel boundary. The model also derives a marginal energy consumption formula for metro operations, capturing microscopic stop-start characteristics. Under the baseline scenario (a single shipment of 55 t over a 40 km O-D (Origin-Destination) distance), M-ULS reduces carbon emissions by 69.8% compared to DT and by 24.2% compared to ET. These advantages are conditional. The emission edge inverts when last-mile drayage exceeds a critical distance of 9.6 km, and reduction rates above 20% require trunk distances beyond roughly 19 km together with drayage shares kept below 15% of total distance. A network-distance analysis of Shenzhen shows that about 79% of freight endpoints lie within a 3 km drayage radius of metro stations, indicating that these boundary conditions are largely attainable in practice. The study further reveals distinctive climate robustness. Under severe congestion and large temperature deviations, whether severe cold or intense heat, the emission reduction dividend of M-ULS relative to electric trucks is maximized. These findings delineate clear spatiotemporal boundaries for M-ULS applicability and provide quantitative evidence for tiered, zone-specific green logistics strategies.