If you ride the New York City subway or bus system and want to be alerted via Twitter when there are service disruptions or planned track work, follow the appropriate link below.
@NYCtrains (all NYC trains)
The MTA provides this info via email, and I had a program written to post the information to Twitter. I created this back when I lived in NYC, and maintain it still. My favorite train is the 7.
Also see: Official NYC Transit Twitter page