Here with your daily dose of infrastructure porn is XKCD's Randall Munroe, who's gone and rounded up all of North America's various subway systems and combined them into one big interconnected map.
(Not to be confused with this metro-inspired map of U.S. highways, which depicts America's Route and Interstate systems in the form of a criss-crossing subway grid). It's lovely. Tangled, colorful and lovely.
For a visual representation of metro system distribution worldwide, see here. For a nice tabulated list, see here. For an explanation as to why certain North American "subway" systems did or did not make it into Munroe's map, see his explanation over on XKCD.