In a weird convergence of form following function, the world's subway systems are marching toward a kind of "unified theory" of structure for the complex operations. The route patterns seem intuitively logical, but they have developed across different cultures, in different geographies and economies, and often piecemeal over decades. The "common mathematical space may hint at universal principles of human self-organization" which we're somehow driven to replicate, notes Wired.
A recent analysis of systems in New York, Tokyo, Barcelona, Paris, London, Seoul, Osaka, Moscow, Mexico, Madrid, Berlin, Beijing, and Chicago uncovered certain trends similar to all. Not only do many of the configurations of routes look similar, but roughly half the stations in all of the subway systems are found on its outer branches, rather than the core. In addition, the distance from a city center to its farthest station is twice the diameter of the subway system's core. “Many other shapes could be expected, such as a regular lattice,” said statistical physicist Marc Barthelemy of France’s National Center for Scientific Research. “What we find surprising is that all these different cities, on different continents, with different histories and geographical constraints, lead, finally, to the same structure.” The evolution to a similar shape occurs organically. The convergence appears to be "a sign that there are some basic, profound mechanisms that drive the development of urban systems,” says Barthelemy.