Mapping the world's subways to scale
There are many ways of mapping subway systems, but they almost always involve bright colors for the different routes, and not-to-scale representations of the systems as a whole. Here, instead, are simple line maps of some of the world's subway systems all drawn to the same scale, http://fakeisthenewreal.org/subway/. While clearly still a work in progress, these maps make it easy to compare the relative length and density of the different systems. There are also some useful comments on the maps at http://flowingdata.com/2012/01/13/world-subway-paths-at-scale/.