Cultivating the Infra-Ecosystem: Ecosystemic Strategies for Under-Viaduct Space Reuse in Japan

Japan's grade separation policy structurally enables below-market community management of under-railway viaduct spaces, yet the social welfare potential of these infrastructural residual spaces remains systematically unrealized in most station areas. Under the 2007 Ministry of Land, Infrastructure, Transport and Tourism guidelines, railway operators bear a minority share of elevation costs and retain leasing rights over up to eighty-five percent of the resulting under-viaduct area. TauT Hankyu Rakusaiguchi in Kyoto represents one of the most fully documented instances. Since the 2015 Comprehensive Cooperation Agreement between Kyoto City and Hankyu Railway, the project has managed approximately 11,200 square meters of under-viaduct space in a newly developing station area under a deliberate 0.5 percent internal rate of return framework. This paper asks what made this model work, and what parameters would allow comparable operators and municipal governments to replicate it. Drawing on stakeholder interviews, a multi-year event dataset of 508 events and 120,754 participants, and accessibility gap analysis across three station areas, the study documents the conditions under which low-IRR management evolves into a self-sustaining infra-ecosystem: an adaptive alignment of product, promotion, and people enabled by catchment population density, co-promotion of social and economic activities, and facility provision targeting identified service gaps. To make this system legible to operators who lack Hankyu's institutional history, the paper introduces social IRR as a measurement framework that stacks monetized community benefits atop commercial cash flows. These findings offer a replicable governance template for under-viaduct activation where the institutional preconditions of public subsidy and multi-stakeholder coordination are present.