This site is cool. It shows you maps of subway tracks (not route maps but track maps) in Manhattan, The Bronx, Queens, and Brooklyn. I love that we are able to have a peek at what goes beyond our sightline while on the subway platform.
This bit is special to me since it shows how the tracks are laid out in the East Village/Lower East Side (my neck of the woods). You'll have to click on the picture to see it in good detail:
If only we could take guided tours of abandoned rail lines!