This study examines the interplay between large-scale rail infrastructure expansion and long-term vegetation dynamics in the Seoul Metropolitan Area, highlighting how integrated urban planning can mitigate or even reverse environmental degradation typically associated with rapid urbanization. Uniquely, Korea's new towns have been systematically developed as transit-oriented districts to alleviate the hyper-concentration of population in Seoul, resulting in a context where railway construction and urban growth are tightly coupled. Contrary to the prevalent narrative of inevitable green space loss, the spatial analysis reveals that several newly developed districts, designed with explicit ecological considerations, demonstrate stable or even increasing vegetation indices over the study period. By employing a combined Geographically and Temporally Weighted Regression framework, we identify localized trajectories where synchronized transit investments and proactive green urbanism policies have yielded net ecological gains. These findings position Korea's policy-driven urban model as an instructive case study for reconciling infrastructure development with ecological resilience, underscoring the critical importance of integrating environmental objectives into transportation and land-use planning.