Don Holbrook of the National Transportation Board said on a Canadian Broadcasting Corp. television program the train, which derailed in northwestern New Brunswick, experienced an "undesired brake application" resulting from a break in a continuous air line, causing the brakes to go into emergency mode.
"Trains have a continuous pipe running throughout the train that supplies air to the brakes system on every car. If that brake pipe comes apart, that causes the brakes throughout the train to go into emergency ... and that means the train will stop as fast as it can," Holbrook explained.
Fire burned through the night into Wednesday after 16 cars of the 122-car CN Rail train hauling crude oil and propane derailed Tuesday evening, fire officials said.
"We have determined that the cars that are affected in the derailment are crude oil and propane cars," Plaster Rock, New Brunswick Fire Chief Tim Corbin said, adding responders had to wait for daylight to "get a better look at it as how we're going to attack it."
CN Rail spokesman Jim Feeny said four of the derailed cars carried crude oil and four held propane.
He said no injuries were reported, but added it is not known if the cars carrying crude oil or propane are involved in the fire.
Hazardous material crews from Toronto, Moncton and Montreal were dispatched to the scene, CBC said.
Fire officials said about 50 homes near the derailment were evacuated.