Subways converging

Wired has an article on the convergence of subway systems around the globe.

Patterns emerged: The core-and-branch topology, of course, and patterns more fine-grained. Roughly half the stations in any subway will be found on its outer branches rather than the core. The distance from a city’s center to its farthest terminus station is twice the diameter of the subway system’s core. This happens again and again.

The findings are taken to indicate that there are underlying principles guiding urban form. This may not contradict their claims, but one must consider that cities have historically grown from a single core and are designed to sustain the value of that core. I also wonder if this isn't a case of cost-effective operation dictating the shape of the machine and urban development following. This, which I think is highly likely, would suggest that the pattern derived from technological design or capitalist concepts of efficiency.