Depopulation challenges the transition to low-carbon transport in Japan’s rural areas

Decarbonization of transport system in Japan requires region-specific policies, as public transport efficiency varies by population density. While rail systems are carbon-efficient in urban areas, their benefits weaken in sparsely populated regions, exacerbated by ongoing depopulation. Here, we integrate spatially detailed transport data, demographic projections, and a model of rail transport under varying trip demand to evaluate the low-carbon advantage of rail relative to cars across regions toward 2035. Our findings reveal rail remains more carbon-efficient than cars in cities, as future grid decarbonization enhances the advantage of electrified rail networks. In rural areas, however, emissions per transport unit can exceed those of single-occupied cars, with grid decarbonization offering little benefit, due to infrequent service and continued use of non-electrified lines. This study highlights that shifting demand to rail is not universally effective for decarbonization. Instead, flexible, small-scale public transport in low-demand areas may better align with Japan’s decarbonization goals.