I have been curious about what started the whole shebang. Here’s the answer.
“At around 2250, the train arrived at Nantes, was brought to a stop using the automatic brakes, and was parked for the night on a descending grade on the main track. The LE applied the independent brakes to the locomotive consist. He then began to apply the hand brakes on the locomotive consist and the buffer car (7 cars in total), and shut down the 4 trailing locomotives. Subsequently, the LE released the automatic brakes and conducted a hand brake effectiveness test without releasing the locomotive independent brakes. The LE then contacted the rail traffic controller (RTC) responsible for train movements between Farnham and Megantic Station (Megantic), who was located in MMA’s yard office in Farnham, to indicate that the train was secured.
The LE then contacted the RTC in Bangor, Maine, who controlled movements of United States crews east of Megantic. During this conversation, the LE indicated that the lead locomotive had continued to experience mechanical difficulties throughout the trip and that excessive black and white smoke was now coming from its smoke stack. The LE expected that the condition would settle on its own. It was mutually agreed to leave the train as it was and that performance issues would be dealt with in the morning.
A taxi was called to transport the LE to a local hotel. When the taxi arrived to pick up the LE at about 2330, the taxi driver noted the smoke and mentioned that oil droplets from the locomotive were landing on the taxi’s windshield. The driver questioned whether the locomotive should be left in this condition. The LE indicated that he had informed MMA about the locomotive’s condition, and it had been agreed upon to leave it that way. The LE was then taken to the hotel in Lac-Mégantic and reported off-duty.
At 2340, a call was made to a 911 operator to report a fire on a train at Nantes. The Nantes Fire Department responded to the call and arrived on site, and the Sûreté du Quebec (SQ) called the Farnham RTC to inform the company of the fire. After MMA unsuccessfully attempted to contact an employee with LE and mechanical experience, an MMA track foreman was sent to meet with the fire department at Nantes. When the track foreman arrived on site, the firefighters indicated that the emergency fuel cut-off switch had been used to shut down the lead locomotive. This shutdown put out the fire by removing the fuel source. Firefighters also moved the electrical breakers inside the locomotive cab to the off position to eliminate a potential ignition source. These actions were in keeping with railway instructions.
Both the firefighters and the track foreman were in discussion with the Farnham RTC to report on the condition of the train. Subsequently, the fire department and the MMA track foreman left the scene.
With no locomotive running, the air in the train’s brake system slowly began to be depleted, resulting in a reduction in the retarding force holding the train. At about 0100 (July 06), the train started to roll downhill toward Lac-Mégantic, 7.2 miles away. At about 0115, the train derailed near the centre of town, releasing about 6 million litres of petroleum crude oil, which resulted in a large fire and multiple explosions.”
http://www.tsb.gc.ca/eng/rapports-reports/rail/2013/r13d0054/r13d0054.asp