Learn a bit of history about the London Underground, namely the schematic layout of the system map that was created in 1933. This map is still in use today, with some changes of course. This particular map allows you to see the original version morph into the current version then into a geographically correct version.
The “schematic” design was used by many other transportation systems around the world. Take a look at the current New York City, Boston, Chicago, and Paris system maps and notice the similarities. I think the map of the DC Metro most reminds me of London’s Underground map. The Potomac River winds through the system in the same mechanical way as the Thames on the Underground’s map.