I love the history that you see when riding the NYC subway.
There are new additions, like the shiny new trains that you see now and then or the electronic signs in some stations that tell you when the next train will arrive. But then there are old elements as well, like the old trains and the low wooden cubby-like benches in some of the stations that you can tell have been there for decades.
Every station is different (giving a very different effect from the DC metro system, where everything is much more uniform). Some have murals and a much more modern look to them, while others still have the station name spelled out in mosaics. If I lived in NYC, it would be an interesting project to photograph all of the stations to see just how different each station is from the rest.
Actually, to see if anyone has done this (I’m sure many people have), I googled “NYC subway photo project,” and the first search result was The New York Subway Project, a blog documenting someone’s project to visit all of the New York subway stations within two weeks in 2007. They even included an interactive map of the subway system so that you can click on the station you want to see. That was a nice touch. I’ll have to remember this site the next time I’m giving tips to a friend visiting NYC. :)