This is really cool! If you're near Grand Central, you should stop into the New York Transit Museum's store and annex at the shop level inside GCT and visit the Transit Etiquette show now on display.
The show not only includes vintage MTA/NYCTA posters ranging from littering to loud music to pole hugging, but also includes posters and signs from other major transportation systems worldwide. As a poster junkie, I want just about all of these -- there's something very design-worthy of them (unlike today's posters which are hideous).
PS: You know what's crazy? The NYC Transit Museum is selling a Subway map for $15. Um, do people not know that these are free?