Here’s what the description says:
MTA Subway Time gives you real-time train arrival information for selected routes of the New York City subway system. Knowing exactly when your train will arrive will reduce the time you spend waiting on the subway platform, and let you know of any service delays or reroutes before you pay your fare.
The app is free. Right now it only encompasses ten subway lines, but this is a huge start. Next, maybe they’ll install some wifi in the subway system so we can actually use the app when we’re underground?