According to Rail Safety Advisory Letters issued by the Transportation Safety Board of Canada on July 19, 2013, the incident is summarized as follows.

At approximately 10:45 p.m. Eastern Daylight Time (EDT) on July 5, 2013, MMA train 2 was proceeding eastward from Montreal, Quebec, to St. John, New Brunswick.  The train was approximately 4,700 feet long and weighed over 10,000 tons.  It consisted of five locomotives, a loaded box car, and 72 loaded tank cars containing petroleum crude oil (U.S. DOT Hazard Class 3, UN 1267).  At approximately 11:00 p.m. the train stopped near milepost 7.40 near Nantes, Quebec.  At that location the operator of the train secured it and departed, leaving the train unattended on mainline track with a descending grade of approximately 1.2 percent.  At around 11:50 p.m. a local resident reported a fire on the controlling locomotive (MMA 5017) of the train.  The local fire department was called and responded with another MMA employee.  At approximately midnight, the controlling locomotive was shut down and the fire extinguished. After the fire was extinguished, the fire department and the MMA employee left the site.  At approximately 1:00 a.m. the next day (the early morning of July 6th) it appears that the train began rolling and picking up speed down the descending grade toward the town of Lac-Mégantic, Quebec, which sits approximately 30 miles from the United States-Canada border.  Near the center of town, the box car and 63 of the loaded tank cars derailed.  The locomotives, which separated from the train, traveled an additional 1/2 mile before coming to a stop.  A number of derailed tank cars released product resulting in multiple explosions and subsequent fires.  At this time, it is estimated that there were 42 fatalities and that 5 persons are still missing.  There was also extensive damage to the town, and approximately 2,000 people were evacuated from the surrounding area.  While the investigation is ongoing and the Transportation Safety Board of Canada has not reached any final conclusions, it has made a determination that the braking force applied to the train was insufficient to hold it on the 1.2-percent descending slope between Nantes and Lac-Mégantic.