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Natural gas flares are seen at an oil pump site outside of Williston, N.D., on March 11, 2013. Oil drillers in North Dakota’s Bakken shale fields are allowing nearly a third of the natural gas they drill to burn off into the air, with a value of more than $100-million (U.S.) per month, according to a study. (SHANNON STAPLETON/REUTERS)
Natural gas flares are seen at an oil pump site outside of Williston, N.D., on March 11, 2013. Oil drillers in North Dakota’s Bakken shale fields are allowing nearly a third of the natural gas they drill to burn off into the air, with a value of more than $100-million (U.S.) per month, according to a study. (SHANNON STAPLETON/REUTERS)

The deadly secret behind the Lac-Mégantic inferno Add to ...

An investigation into the disaster and its causes.

The shrieking whistle of escaping gas continued for hours.

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Emergency crews ran for cover when they heard the noise, as they fought blasts of burning oil during the Lac-Mégantic rail disaster. The kettle-boil scream meant one thing: Oil vapours were shooting out of a derailed tank car and another fireball was about to rip from the broken train.

It wasn’t until four days after the July 6 derailment that the fires finally subsided. But even before the inferno was extinguished and the burned-out town counted its 47 dead, rescue workers and rail, petroleum and government officials were asking the same troubling question: Why was the oil so explosive?

The North Dakota crude that levelled Lac-Mégantic was classified as flammable, a long-standing practice for all oils moved by rail. Hazardous material experts and rail officials interviewed by The Globe and Mail say the risks of exploding crude were not scrutinized until the tragedy.

“The explosions and everything, I didn’t think crude oil did that,” said Ed Pritchard, a former accident investigator with the U.S. Federal Railroad Administration.

Related video: How oil from the Bakken formation decimated Lac-Mégantic

Canada’s Transportation Safety Board agreed. During an August briefing on its investigation into the crash, Ed Belkaloul, head of the federal TSB in Quebec, said the oil carried to Lac-Mégantic is undergoing testing because the crude reacted “in a way that was abnormal.”

The potential explosiveness of the crude should not have been such a mystery. An investigation by The Globe into the Lac-Mégantic explosions shows there were warning signs that crude from the Bakken region straddling North Dakota and parts of Manitoba and Saskatchewan was not like other oils.

In New Town, N.D., where the ill-fated train was loaded with Bakken crude, locals like to boast that the honey-coloured oil is so light they can take it right from the well and pour it into truck engines because it requires little refining. Long before the crude exploded at Lac-Mégantic, there were signs that shippers, regulators and rail officials did not appear to consider the variable characteristics of oil loaded onto trains that travel through towns and cities.

As early as 2010, North Dakota geologists were investigating lethal gases in the oil. Early last spring, the Washington-based FRA grew concerned about Bakken crude when it became aware of “severe corrosion” of tank car walls and joints, according to a letter sent to a U.S. petroleum association. The five-page letter, a copy of which was obtained by The Globe, cites test findings showing that some Bakken oil was so flammable it could be ignited at temperatures as low as 20 C.

Bigger alarms went off in May, when pipeline giant Enbridge Inc. discovered dangerous hydrogen sulfide levels in Bakken crude – 24 times the legal limit. That was enough to prompt the U.S. Federal Energy Regulatory Commission to grant emergency powers to Enbridge to block crude that carried excessive amounts of the potentially lethal and explosive sulfide vapours from entering its pipeline.

Bakken oil is potentially more hazardous than conventional crude because it is lighter and contains a number of gases and compounds, such as methane and propane, that can make it much more corrosive and volatile.

What no one appears to have responded to was the safety of the rail transportation network carrying the oil. Today, railways haul more than two-thirds of the nearly 700,000 barrels of crude shipped daily from the Bakken formation of shale oil. Crude initially stranded by a pipeline shortage is now carried on trains loaded with as many as 120 cars of oil – so-called unit trains of crude that didn’t exist five years ago.

U.S. and Canadian railway laws require shippers, railways and even buyers to ensure hazardous goods such as petroleum are properly classified so they are carried in sufficiently sturdy cars. But interviews with rail officials and exclusive access granted to a Globe reporter to the North Dakota crude facility that loaded the Lac-Mégantic train indicate that the makeup and flammability of Bakken oil is not always tested.

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