I just saw a very interesting post with scaled maps of major subway/metro systems of the world over at fakeisthenewreal.com.  BART appears to be the largest system in the world by reach.  BART is sort of a hybrid between a traditional metro system and a commuter rail system. I’m never sure if its the best of both worlds or the worst. A few examples are below for comparison.

Paris

New York

London

San Francisco BART

See many more at Fakeisthenewreal.com.

I’m not really sure what that line is heading down Market St diagonally in the western section of the BART map, but either way it appears to be larger than all of the others. I wonder what all of these maps would look like if you included commuter rail lines as well.

Thanks to the Transportationist for pointing these out.