UPDATE, 4:30 p.m. The T reports regular service has resumed.
The T is warning Blue Line passengers to expect "significant delays" this afternoon following a train slipping off the rails at Orient Heights.
MBTA spokesman Joe Pesaturo reports the train was moving from the Orient Heights train yard onto the main tracks around 2:40 p.m., so it wasn't carrying any passengers. The T has swapped in buses for trains between Suffolk Downs and Airport.