These maps of unrealized city plans, collected by Andrew Lynch, provide a sort of Borgesian vision of alternate-universe cities that never came to fruition.
Above is a plan for a 1945 Boston rail expansion into the suburbs that never happened.
And a 1925 plan for rapid transit in Los Angeles that seems downright laughable now.
On the flip side, transit paradise Manhattan could have had a giant, futuristic expressway running through it, as this awesome-looking 1960s proposal sketch shows:
You can see more at Wired or on Lynch’s Tumblr, Hyperreal Cartography & The Unrealized City.