It's New York City day here at The Map Scroll! Here's a link to an animated map that shows the growth of the NYC subway over time, from Appealing Industries (via Spacing Toronto).
Unfortunately there's no time legend, which would have seemed like a no-brainer to include. Still a very interesting animation, though. And if you want to see about eleventy billion more maps from throughout the history of the NYC subway, go here.
And as a bonus, here's another one from Spacing Toronto of Toronto's own (admittedly less exciting) subway map.