AIGA has a great (lengthy) article about the type used throughout the history of the rail system in NYC. Today, Helvetica is the standard used throughout the system, but it wasn’t until almost the 90s that this was so. Great read if you have some time.
[the link above is for the "print" verison of the article, so you can read it all on one page. To see it in its original verison, you can visit the AIGA website.]