BBC News - Subways 'share universal structure', research suggests: In some senses, it is unsurprising that the study found that networks tended over time to comprise a dense core of central stations with a number of lines radiating outward from it...
But the analysis shows a number of less obvious similarities across all 14 networks.
It found the total number of stations was proportional to the square of the number of lines - that is, a four-fold increase in station number would result in a doubling of the number of lines.
The dense core of central stations all had the same average number of neighbours in the network, and in all cases, about half the total number of stations were found outside the core.
In addition, the length of any one branch from the core's centre was about the same as twice the diameter of the core.
The number of stations at a given distance from the centre was proportional to the square of that distance, but only up to the edge of the core; at more distant reaches of the network, the number of stations contained was directly proportional to distance...
The authors say that the systems do not appear to be "fractal".