Figure 5.

Schematic of subway networks. A large ‘ring’ encircles a core of stations. Branches radiate from the core and reach further areas of the urban system. The branches are essentially characterized by their size (parameter β(t), equation (3.1)), and their spatial extension (parameter η(t) in equation (3.2)). The core is characterized by its average degree ( defined in equation (3.3)) and fraction of nodes of degree 2 (f₂), its number of stations N_C(t) and its size r_C(t). Green circles denote junction; red, terminus. (Online version in colour.)