Which Mode Clearly Shapes the Spatial Linkage of Urban Agglomeration, Rail, or Others?—A Case Study of Beijing–Tianjin–Hebei (J-J-J) Urban Region

Intercity transport infrastructure provides a framework for spatial linkages among urban agglomerations. This paper identifies the dynamic changes of spatial correlation degree of Beijing–Tianjin–Hebei (or “J-J-J”) urban agglomeration by calculating core entropy value and traffic connection degree. The result shows that, first, the urban spatial layout, urban economy, spatial connection, and transportation network within the urban agglomeration are closely linked. During the expansion of the transportation networks, their impact will be far greater than that of the geographical location. Second, the traffic network greatly changes the spatial layout of the urban agglomeration, thus reshaping the connections between cities. Third, the transportation network furthers the evolution of regional integration. This paper makes up for the one sidedness of existing research and provides a valuable perspective for the spatial research of urban agglomerations.