A paper in the latest issue of the Royal Society journal Interface details how researchers at France’s National Centre for Scientific Research have carried out a mathematical analysis of 14 of the world’s largest subway systems. They discovered that despite differences of geography and economics and scale and a lack of co-ordinated central planning, they all appear to be converging on the same overall network structure. Some similarities are only to be expected – a core with lines branching off it may be obvious as well as functional, and later transport engineers are likely to have been influenced by the work of earlier ones – but there are others which are harder to explain. Such as all the different subways where the distance from the city centre to its furthest station was twice the diameter of the system’s core. Or the total number of stations again and again being proportional to the square of the number of lines.
There are suggestions that at the very least this is evidence of urban evolution and self-organisation, and that it could even help us to build cities which have the capacity for self-improvement. Perhaps. In the shorter term the idea of an ever-evolving network of tunnels sounds like a scary subterranean variant on the changing corridors and stairs of Hogwarts or the hotel in recent Dr Who episode The God Complex. So is that art mimicking life or something deeper?