The BBC and Scientific American report on a paper looking, “in an exploratory manner,” at the limiting shape of metro systems serving large cities. The BBC linked to the actual paper, which is nice of them. The Scientific American article goes into a bit more detail, though.
The authors contend that rather than the shape of subway networks being decided by central planning, which would produce a variety of shapes, the eventual shape of a subway network converges on an emergen structure consisting of a dense core with branches radiating from it.
I’m not a statistician, so I can’t really tell what they’ve done, but they seem to have tried out a variety of metrics and statistics on their data which showed that all networks converge on this core-and-branches structure.
The key finding is that some fundamental aspect of how metro systems work guides the evolution of their structure, regardless of the political and geographical context.
I’m not entirely sure why this was picked up by the mainstream news – was it a hard-working university PR department, or the easily-understood subject matter?
BBC - Subways ‘share universal structure’, research suggests
Scientific American - Track Record: Do Major Urban Subway Networks Evolve along Similar Patterns?
Paper: A long-time limit for world subway networks - Journal of the Royal Society Interface, not open access.