Topological Vulnerability of Subway Networks in China

Modern cities depend on the operations of urban subway systems. Their networked nature permits its objective representation using tools from graph theory. We model subway networks as graphs, and present the empirical investigation on ten existing and planning subway networks (SNs) of major cities in China. In SNs, nodes are stations, edges are transmission lines. The empirical results show that every subway networks studied has Poisson degree distribution and most of them lack typical small-world topology. Further, values of topological characteristics of the networks, i.e., the relative size of the largest connected component, are numerically calculated to compare their error and attack tolerance (topological vulnerability), i.e., their performance when vertices are removed. It is found that the removals by the recalculated degrees and betweenness are often more harmful than the attack strategies based on the initial network, suggesting that the network structure changes as important vertices are removed. We conclude that, actual planning strategies to increase these systems would have to take into account this vulnerability, in order to facilitate and improve the subway network design and function.