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.