I’m trying to be a good tourist (and history major) and read up on the things I will visit when I go to Washington DC. I’ve found a very nice site on the history of the Metro. It isn’t super wordy, and has a trove of great images, such as the one above which was a bond referendum poster. You can read all about the history of this transit system by going to: http://chnm.gmu.edu/metro/index.html
It’s especially fun to compare the planning maps to the current one.
image from: http://chnm.gmu.edu/metro/popup/bond68.html