Derailment happened at the major interlocking that starts at the I-495 overpass in South Lawrence and encompasses all of the tracks that split off for the Lawrence freight yard. Very mission-critical area because of the punishing amounts of traffic that go over it all day and night, and maybe the northside's single most disruptive point-of-failure outside the terminal district due to all the traffic that gets stopped up around the system when there's a problem at that very spot: Haverhill revenue trains, all Ayer-Portland freight, Downeasters, every Boston-north local freight (Lawrence is Pan Am's originating yard), the work trains doing heavy construction on Haverhill Line and Merrimack River bridge, and non-revenue deadheads to restock Bradford layover yard.
It was icy on that swath of 495 yesterday morning, so could be anything from an ice-gunked split switch to frost heave from the extremely sudden ground freeze (which we didn't get in December). Overnight the activity around Lawrence is all-freight and stops over in the yard instead of blowing by on the mainline, so this non-revenue train would've been the first MBTA activity since midnight and first train on that particular track since midnight. If something temperature or ice related happened to the track this would've been the first train to find out. That's why they do deadheads or hi-rail track inspections about an hour before the first revenue train on most lines. Stuff does go bump in the night. This just happened to be one of the spots where it's hardest to triage around an overnight maint issue in time to prevent mass-hosing of the A.M. commute.