A broken drawbridge.
From the T web page:
Newbury/Rock #168 (11:06am ib) experiencing over 30 min delays due to Saugus Drawbridge stuck open. 11/19/2009 12:12 PM
So the train left at 11:06, and the alert was issued at 12:12. Yah, I'd say that's well over 30 minutes.