Smoke and steam were still coming from the accident scene in a rural setting on Thursday morning.
CN Rail spokesman Jim Feeny told CBC's Information Morning in Fredericton that CN's preliminary investigation indicates there was a sudden failure of a wheel, or perhaps a wheel-axle combination on Car 13, which was the first car that derailed toward the front of the 122-car train.
"That then led to application of the emergency braking system, which is exactly what it's designed to do," said Feeny. "If there is a mishap of this sort on a train, the emergency brakes are designed to apply and then bring the train to a stop.
"However, as that was happening, the further 16 cars and locomotive combination [derailment] occurred towards the end of the train. That's where we had the fire."
Among the 16 cars that derailed at the end of the train were five cars carrying crude oil and four cars filled with liquefied petroleum gas.
Each tank car can carry between 550 and 650 barrels of oil, according to the Rail Association of Canada.
Feeny said there have been derailments in the past involving the failure of a train wheel, or wheel-axle combination.
Investigators will now need to determine the sequence of events, he said.
"What happened first? Was there a problem with the wheel? Did that lead to a problem with the axle?" he said.
"We have a preliminary indications. What we don't have is a definitive analysis of the chain of events yet. That will take some time."
The federal Transportation Safety Board said it is eager to get a closer look at the wreck Thursday.
The TSB is in charge of the investigation. It has a news conference scheduled for Plaster Rock on Thursday to give more details about the investigation.
Air and ground assessments of the area are expected to continue.
The CN freight train jumped the tracks and caught fire late Tuesday near the village of Plaster Rock.
The train was headed east from Toronto to Moncton, N.B.
The crude oil was from Western Canada, said Claude Mongeau, president and CEO of CN Rail, at a news conference in Plaster Rock on Wednesday.
The crude oil and propane were destined for the Irving Oil refinery in Saint John, he said.
It is not known if the crude oil was heavy crude, or the potentially more explosive Bakken crude, which comes from North Dakota and southern Saskatchewan and Manitoba.
Bakken crude was involved in the massive derailment and explosion in Lac-Mégantic, Que., in August 2013 that killed 47 people. That shipment was also destined for the Saint John refinery.
Aerial images from the scene near Plaster Rock show a jumble of burning cars strewn across the tracks in a wooded area.
No one was injured, but about 150 people living in nearby homes were told to leave soon after the train derailed.
54 derailments in N.B. since 2003
Fifty-four trains carrying dangerous goods have derailed across New Brunswick over the past decade, CBC News has learned.
The 2003-12 figures come from a TSB database of reported rail occurrences obtained by CBC News as part of an ongoing investigation into rail safety.
Nearly a quarter of those reported derailments involving dangerous goods cars — 13 in total — happened in what’s known as the Napadogan subdivision, an area that includes the tracks where yesterday’s derailment took place. However, the subdivision hasn’t had a dangerous goods car derailment for the past five years, since 2007.
Across New Brunswick and on the Napadogan subdivision train lines, there has been a decrease in the number of derailments and occurrences in general over the past decade.
At least two other derailments have happened near Plaster Rock, one in 2004 and another in 2005. Both involved CN Rail cars carrying dangerous goods. One involved petroleum gases, while in the other case the product type is not stated.