Not to nerd out, but this is the future. BART (SF subway) opened all their data to developers... which means anyone can build apps that help people Miss Trains Less Often. Here's an XML feed of what trains are arriving when: (feed updates every 60 sec)

... basically this is the type of data that drives the signs you see on the L train ("Next Train: 25 minutes"). Now imagine building this into an iPhone app or something so whenever you are standing over a subway station (GPS!) the phone can tell you whether it's worth going in and paying the $2 (vs. you sitting around waiting 45 mins for the next train).... or your phone buzzing with an SMS before you leave your apt for work / airport/ night out letting you know the F train is on fire / delayed 20 minutes / etc

Unfortunately, I don't think the MTA has this type of data for all their trains, but this is where the space is going. Awesome, eh?

More info on BART's API here:
www.bart.gov/schedules/developers/

(btw, I even love the casual language they use to explain how the API works. Well done, BART)