Against the backdrop of urban rail transit shifting from large-scale expansion to quality improvement of existing assets, increasing uncertainty in land and property development revenues, and rising long-term operational pressures, the notion that "building rail transit is building the city" has become an urban governance proposition during the transition from intensive rail construction to long-term operation. This paper takes the 30-year development of Shenzhen rail transit as a case study and adopts qualitative interviews based on first-hand materials and case analysis to examine how rail transit is embedded in urban spatial organization and governance processes, thereby being transformed into a form of city-building capacity. Based on the correspondence between different stages of rail transit development and urban transformation in Shenzhen, the study systematically analyzes the functional mechanisms of rail transit in terms of public-interest positioning, network organization, station-area functions, and coordinated implementation, and further examines the operational logic of the "building rail transit is building the city" model. The findings suggest that the urban value of rail transit is not directly derived from network scale or property-led development along rail corridors, but is progressively realized through improved accessibility, spatial restructuring, and interdepartmental coordination; the key to the operation of this model lies in the recirculation of spatial value generated by public investment and benefit-sharing mechanisms, rather than in simple market-oriented development activities. In the context of existing-asset-oriented development, the continued role of rail transit in "building the city" should shift from scale expansion and development-revenue orientation to improving the efficiency of existing networks during the stable operation phase, station-area renewal, and strengthening public service provision.