Now this is pretty cool (and a little depressing for some of us). Neil Freeman has a nifty set of subway maps, all at the same scale, of a bunch of major cities. The contrast is striking.
and BART, (San Francisco) Bay Area Rapid Transit:
The Bay Area does have a smattering of other rail systems—SF Muni, VTA Light Rail, Caltrain—but they don’t dramatically change the picture. (And they aren’t subways, of course, though neither is BART for most of its trackage.)