With equations used to study two-dimensional spatial networks, the
class of network to which subways belong, the researchers turned
stations and lines to a mathematics of nodes and branches. They
repeated their analyses with data from each decade of a subway
system’s history, and looked for underlying trends. 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.
<http://www.wired.com/wiredscience/2012/05/subway-convergence/>
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