More than four years after a runaway train exploded in flames and destroyed the centre of the town of Lac-Mégantic, Que., on July 6, 2013, killing 47 residents, the first witnesses have testified at the trial of three men charged with criminal negligence.

Images of train cars covered in oil, scorched rail ties, and smoke billowing over Lac-Mégantic's downtown core were among the first pieces of evidence presented to the jury.

The trial began with the three accused solemnly reiterating their pleas of not guilty in front of Superior Court Justice Gaétan Dumas, the jury and a courtroom full of reporters.

The three former Montreal, Maine and Atlantic (MMA) Railway employees on trial are:

Thomas Harding,56, the train engineer and conductor on the night of the disaster.

Jean Demaître, 53, the operations manager for the railway company.

Richard Labrie, 59, the railway traffic controller.

The three men are each charged criminal negligence causing 47 deaths.

Steven Monteambeault, a former police officer and crime scene investigator on duty the day of the explosion, was the first witness to testify.

He presented the first piece of evidence, a 13-minute video he took from a helicopter hours after the explosion. Flames were still visible in the main zone.

The next witness was crime scene technician and Sûreté du Québec officer Jacques Lafrance, who was tasked with documenting the scene and assisting investigators.

Lafrance described to the court the dozens of photographs he took of the wreckage, explaining photo-documenting the disaster had been part of his job, along with gathering evidence.

Crown outlines its case

As the three accused sat in the Sherbrooke courtroom, taking notes and sipping water, Crown prosecutor Véronique Beauchamp outlined how she and her colleagues will attempt to prove the three men are responsible, either by an act they committed or by certain omissions, for the runaway train and explosion.

Beauchamp told the jury the Crown will prove all three men were aware of a mechanical problem with the train before the derailment, yet each failed to take the necessary precautions to secure it.

She will play audio and video recordings of the three men in the hours leading up to the tragedy, and will present a total of 36 witnesses, she said.

Earlier, Dumas gave lengthy instructions, lasting almost 45 minutes, in both English and French to the four women and 10 men sworn in as jurors last week, telling them to remember that MMA is not on trial.

"These people are not accused of acting together to commit criminal negligence,'' he went on, saying the proceedings will operate as three distinct trials for the co-accused.

Dumas also warned the jury of the importance in keeping their conversations confidential, and to not form an opinion before they hear all of the evidence.

Fire from a train explosion is seen in Lac-Mégantic on July 6, 2013. The centre of the small Quebec town was destroyed and 47 people died. (Reuters)

Several residents of Lac-Mégantic made the trip to Sherbrooke for the opening of the trial.

Proceedings are scheduled to run until just before Christmas.

Thomas Walsh, the defence lawyer for Harding, said his client would probably take the stand at some point during the trial.

Walsh said he was happy with the first day of the proceedings and that he believed the prosecutor had been fair in her opening remarks.

"We're very optimistic. Mr. Harding, as you can understand, is very anxious and he has been waiting for this for a very long time, and he's happy it has finally arrived."

3 weeks to screen jury

It took three weeks to screen more than a thousand potential jurors for their ability to speak and understand both English and French, assess their impartiality toward the accused and grant exemptions.

The jurors who will ultimately decide the fates of the three men were sworn in on Friday.

Walsh said he is very happy with the jury, and that his client is anxious for the proceedings to get underway.

"He's afraid like anyone else would be, but he's happy it's finally going to be over one way or the other," said Walsh.

The maximum sentence for a charge of criminal negligence causing death is life in prison.