Train driver Thomas Harding knows he's partly responsible for the Lac-Megantic rail disaster that killed 47 people, but he did what he was told to do and the tragedy flowed from bad company policy, his lawyer said Monday on the first day of trial.

"Mr. Harding realizes he's partly responsible for a very serious tragedy and that weighs on him a lot more heavily than the trial," his lawyer Thomas Walsh told reporters.

"Is human error criminal negligence? That's what this case is about."

Harding and two other ex-railway employees, traffic controller Richard Labrie and manager of train operations Jean Demaitre, are all facing one count of criminal negligence causing the death of 47 people.

All three have pleaded not guilty.

Crown prosecutor Veronique Beauchamp told the 14-member jury in her opening statement that if it wasn't for the negligent actions and omission of the three accused, the deaths of 47 people in Lac-Megantic would not have happened.

The oil-laden locomotive weighing more than 10,000 tonnes was not properly secured the night of July 5, 2013, leaving it resting precariously on a slope, 10 kilometres away from downtown Lac-Megantic, Beauchamp told the court.

A fire broke out on the train about 30 minutes after Harding left for the night.

Around 1 a.m. on July 6, 2013, roughly one hour after firefighters put out the flames and turned off one of the engines, the train began moving, picking up speed and barrelling into the town.

All three men, due to their job descriptions, were responsible to ensure the train was safe, Beauchamp said.

"Evidence presented will show that beyond a reasonable doubt, all three were criminally negligent ... they contributed to the deaths of the 47 victims," she said.

"You'll see that the number of brakes applied to the (train) were clearly insufficient," Beauchamp added.

She said one of the accused, supervisor Demaitre, was even told of the train's mechanical deficiencies before the derailment.

"Our position is that Harding has a degree of responsibility but that doesn't amount to criminal negligence," Walsh said.

"He did what he was told to do. Most of what he did was company policy. That was the situation from which the tragedy flowed."

The trial's first witness was retired Quebec provincial police officer Montembeault, who showed the jury aerial footage he took from a helicopter about 15 hours after the derailment on July 6, 2013.

Light from the setting sun was glimmering on the lake, surrounded by green hills and lush grass of the picturesque town, 250 kilometres east of Montreal.

Then the camera pans to what was left of the centre of the village, and zooms in on firefighters putting out pockets of fire across downtown Lac-Megantic as thick plumes of smoke billowed into the air and could be seen for kilometres.

Following Montembeault was Jacques Lafrance, of the provincial police, who began showing jurors crime scene photos he took of the disaster.

The Crown has signalled it will call 24 civilian and 11 police witnesses, and one expert witness in a trial that is expected to last until December.

Beauchamp said jurors will also be shown video and listen to audio recordings of conversations between railway employees the night of the derailment.

The bankrupt former railway company Montreal Maine and Atlantic Railway has also pleaded not guilty to causing the deaths of 47 people and will face a separate trial at a later date.

The trial is being held in Sherbrooke, 150 kilometres east of Montreal.