Those nifty roller-coaster-like rails in the new Bioshock are called sky-lines. Characters can ride around the city on these things using a nifty tool called a sky-hook. This included video contains a bunch of gameplay footage showing freedom of using these awesome things.
It's rare that we see genuine innovation in FPS games these days. Everything has been done, or so we tend to think, and when someone comes up with some kind of new gameplay mechanic, it rightfully just blows everyone's mind. I gaurantee that you will see some kind of clone of this mechanic in future games, I'm calling it now.