OTTAWA - The question of how the brakes were set — or possibly reset — on a runaway train at the heart of the Lac-Megantic devastation emerged Monday as a critical aspect of the tragedy.

Details and the sequence of events began to take shape with the acknowledgment the locomotive suffered an engine fire just hours before the train derailed Saturday.

The blaze, which was quickly extinguished and thought to have been fed by the engine diesel, may have set in motion the subsequent disaster, experts and the company suggested.

Firefighters were forced to shut down the locomotive in order to combat the fire, depriving the train's brake system of its stable air compressors.

But that event by itself should not have cut the train loose, had a manual system also been activated.

"If you properly set your brakes, you can shut off the engine to the train," said Allan Zarembski, a professor in railroad engineering and safety program at the University of Delaware.

"There are several different ways to set brakes on a train. The most secure way is to set the air brakes and the hand brakes. If you set the air brakes and the hand brakes, you can shut off the train and walk away from the train and nothing should happen."

Specifically, if the devices at either end of the train which prevent air from being lost from the brake pipe — known as angle cocks — are left open, the train should remain stationary, experts said.

The chairman of Montreal, Maine & Atlantic Railway said Monday that shutting off the engine caused the brakes to fail.

"This accident may have been a very strange set of circumstances," said Chris Damas, a transportation expert at BCMI Research.

"Some of the procedures for locking down the train are going to be looked at," said Damas, who warned it may be too soon to begin debating the merits and drawbacks of moving crude oil by rail.

Most of the designated Class 1 railways, such as Canadian National (TSX:CNR) and Canadian Pacific (TSX:CP), have been hauling an increasing amount of oil, largely without incident, he said.

There are questions to be asked about what kind of expertise the regional carrier may have had with delicate cargo, he said.

"You have this train on a secondary rail which may have had less competence in chemicals and (hazardous materials) and crude," said Damas.

"It's a new business for them and so they weren't used to worrying about crude oil. They may not have had some procedures that would have been as automatic as a CN-CP, or Burlington."

Current federal legislation covers many of the safety procedures related to the accident, so it is not a matter of filling a regulatory hole, as some critics have suggested, Damas added.

Brake failure was cited by the country's transportation safety board in a runaway train incident in Edmundston, N.B., 18 years ago.

An eastbound CN freight train had halted at the town yards in December 1995 and the crew uncoupled while a second locomotive was hooked up.

Investigators found they had closed the angle cocks and not applied the hand brakes.

The 15 rail cars ended up rolling westward, uncontrolled, for about 1.4 kilometres through the main track switch, coming to a stop upright with no injuries.

With files by Murray Brewster in Ottawa and Sidhartha Banerjee in Montreal

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  • Work continues at the crash site of the train derailment and fire in Lac-Megantic, Que., on Tuesday, July 16, 2013, that left 37 people confirmed dead and another 13 missing and presumed dead.

  • Work continues at the crash site of the train derailment and fire Tuesday, July 16, 2013 in Lac-Megantic, Que. that left 37 people confirmed dead and another 13 missing and presumed dead.

  • Work continues at the crash site of the train derailment and fire Tuesday, July 16, 2013 in Lac-Megantic, Que. that left 37 people confirmed dead and another 13 missing and presumed dead.

  • A fireman walks through the debris as work continues at the crash site of the train derailment and fire Tuesday, July 16, 2013 in Lac-Megantic, Que. that left 37 people confirmed dead and another 13 missing and presumed dead.

  • A levelled building is seen in the foreground the day after a train derailed causing explosions of railway cars carrying crude oil Sunday, July 7, 2013 in Lac Megantic, Que.

  • Canadian Prime Minister Harper, right, surveys the scene Sunday, July 7, 2013 in Lac Megantic, Quebec, as firefighters continue to spray derailed tanker cars. A runaway train derailed Saturday causing explosions of railway cars carrying crude oil and destroyed part of the downtown area of Lac Megantic.

  • Prime Minister Harper, centre, talks with firefighters Sunday, July 7, 2013 in Lac Megantic, Quebec. A runaway train derailed Saturday exploding tanker cars carrying crude oil and destroying part of the downtown area of Lac Megantic.

  • Prime Minister Stepen Harperwaves to people as he visits the site Sunday, July 7, 2013 in Lac Megantic, Quebec, where a runaway train derailed Saturday. Tanker cars carrying oil exploded after the derailment and destroyed the business district in Lac Megantic.

  • Firefighters keep watering railway cars the day after a train derailed causing explosions of railway cars carrying crude oil Sunday, July 7, 2013 in Lac Megantic, Que.

  • The downtown of Lac Megantic, Que. is seen Sunday, July 7, 2013, the day after a train derailed causing explosions of railway cars carrying crude oil.