Two of the cars that derailed in a fiery New Brunswick train crash were older model tankers that have been criticized as more prone to rupture during an accident, Canadian National Railway Co. says.

The accident has raised new questions about the safety of shipping potentially volatile light crude by rail. It is also placing added pressure on governments to come up with new regulations for the DOT-111 tank cars, which are commonly used to haul crude oil in North America.

It was not immediately clear what type of oil was involved in the accident, but a spokesman from the Transportation Safety Board said on Thursday that the composition of the oil could play a role in the agency's review of what happened. A Globe and Mail investigation found that oil from the Bakken area, which covers North Dakota and parts of Manitoba and Saskatchewan, is lighter and more volatile than traditional crude.

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Oil from the region has been under increased scrutiny since the derailment of a train in Lac-Mégantic, Que., caused explosions that killed 47 people this past July. And during the weeks before the New Brunswick accident, trains loaded with Bakken oil caught fire in Alabama and North Dakota, prompting U.S. officials to issue an alert about crude from that region.

A CN spokesman said three of the derailed cars were loaded at a facility in southwestern Manitoba and filled with oil from Saskatchewan and Manitoba wells. Oil in that region consists both of Bakken crude and crude from other formations and is often mixed before it is shipped – a process that can make it difficult for shippers and railways to know exactly what they are moving and how dangerous it might be.

"If, during the course of the investigations, investigators identify a need, they will evaluate the contents of tank cars to ensure that they are properly classified and the appropriate means of containment were used," Transportation Safety Board spokesman John Cottreau told The Globe and Mail.

"Two of the tank cars of crude oil that derailed in the New Brunswick incident are the older DOT-111 tank cars that CN and the rail industry are recommending be phased out or retrofitted," CN spokesman Mark Hallman said on Thursday. He pointed out that most of those cars belong to shippers or leasing companies, not the railways.

CN said Thursday that three of the five cars that were carrying crude were DOT-111s that had been built to higher standards than the earlier version.

U.S. Transportation Secretary Anthony Foxx told North Dakota senators on Thursday that he will call on railway CEOs and the head of the American Petroleum Institute to explain what they are doing to improve safety while moving crude oil – particularly crude that comes from the Bakken region.

The Department of Transportation said on Thursday that the U.S. government is still looking at the possibility of new regulations, including for tank cars that are used to carry crude oil by rail.

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A spokeswoman for Transport Minister Lisa Raitt said the minister has asked for recommendations on more enhancements to the current DOT-111 tank car rules. "The minister is working closely with Secretary Foxx and our US counterparts on the DOT-111 standards,"Ashley Kelahear wrote in an e-mail.

On Thursday, NDP Leader Thomas Mulcair called for the federal government to take a greater role in ensuring rail safety by increasing the number of safety inspections and relying less on rail companies' reports about their own safety records.

"These companies are not going to do anything more than what's required of them by the government," he said. "What we should be doing is inspecting them ourselves."

Investigators from the Transportation Safety Board said Thursday that they believe one of the train's wheels had a crack that caused it to slip toward the centre of the tracks. That "sudden wheel failure" may have resulted in the derailment of more cars further back on the train, according to CN.

A total of 19 cars carrying crude oil, liquefied petroleum gas and other goods slid off the tracks on Tuesday night, sparking a fire that was still burning on Thursday and forcing about 150 people from their homes in the nearby town of Wapske.