The London Underground

The train-based part of London's public transport system. Generally not as bad as the papers would have you believe, in my experience. Also known as The Tube?. There's a very cool 3D map here: http://www.recenda.f9.co.uk/pages/tubemap.htm

And here's a version with walklines: http://rodcorp.typepad.com/photos/art_2003/tube_walklines_final_lm.html

Someone has also done a map tube-style to demonstrate the road-links into and out of London. With the M25 substituting for the Circle Line, obviously. See Circle Line Party.