A judge urged jurors in the Lac-Megantic trial on Tuesday to try once more to reach unanimous verdicts after they told him they had come to an impasse on Day 6 of deliberations.

Quebec Superior Court Justice Gaetan Dumas told the eight men and four women that failing to reach verdicts for the three accused will not reflect badly on them, provided they "made an honest effort."

"The law gives me the power to dissolve the jury if it appears that holding you longer would be useless," Dumas said. "This power can't be used lightly or prematurely.

"Will you please try once again to reach a verdict? This is a time for each of you to reflect further on the evidence and to see how, listening to each other carefully and reasoning together, you can come to an agreement."

The jurors are deliberating the fate of Tom Harding, Richard Labrie and Jean Demaitre. The three were charged with criminal negligence causing the 2013 tragedy that killed 47 people when a runaway train carrying crude oil derailed and exploded.

After Tuesday's lunch break, Dumas convened the prosecution and the defence teams back into the courtroom and read a letter from the jury.

"We are at an impasse," Dumas said, referring to the letter. "What happens if we can't arrive at a unanimous decision?"

Before the jurors began deliberating last Thursday, Dumas told them the verdict for each of the accused had to be unanimous.

All three men can be found guilty of criminal negligence causing the death of 47 people, while jurors have the option of convicting Harding on one of two other charges: dangerous operation of railway equipment or dangerous operation of railway equipment causing death.

Harding was the train's engineer, Labrie the traffic controller and Demaitre the manager of train operations.

The three men each pleaded not guilty to one count of criminal negligence causing the death of 47 people.

None of them presented a defence at the trial, but lawyers for each told the jury in turn the Crown had failed to meet its burden of proof.

The prosecution mounted a case that the three were each criminally negligent in their own way for failing to ensure the train was safe before the wee hours of July 6, 2013.

That's when the locomotive and its cargo of crude oil from the United States rolled away and derailed in Lac-Megantic, exploding and then killing the 47 people as well as destroying part of the downtown core.

The Crown argued that Harding's role was a significant one because he didn't apply a sufficient number of brakes after parking the oil-laden convoy for the night in nearby Nantes.

That left the locomotive, which weighed more than 10,000 tonnes, resting precariously on a slope 10 kilometres away from downtown Lac-Megantic.

Harding applied only half the required level of brakes and didn't test them to ensure they worked properly before leaving for the night.

One of Harding's lawyers countered that the rail disaster was an accident resulting from a perfect storm of unforeseeable events.

"We can't hold people criminally responsible for not being perfect," Charles Shearson told the court.

Shearson said Harding admitted to not conducting a proper brake test and failing to apply a sufficient number of handbrakes, which would have prevented the train from moving after its engine was shut off.

He suggested that evidence presented during the trial demonstrated the Montreal, Maine and Atlantic railway didn't require its employees to perform brake tests perfectly in line with the federal regulations.

And Harding could not have foreseen the locomotive would catch fire after he left for the night, the lawyer added. Firefighters extinguished a blaze at the lead locomotive shortly before the tragedy and cut the engine, which meant the air brakes were not functioning.

The prosecution also blamed Labrie and Demaitre, arguing their responsibilities included taking the necessary steps to avoid injuries and loss of life the night before the derailment.

The prosecution claimed neither man deemed it necessary to check with Harding to see how many handbrakes had been applied and whether tests had been conducted.

Demaitre's lawyer, Gaetan Bourassa, argued his client had no say in safety-related decisions made in the United States by Montreal, Maine and Atlantic's parent company.

Labrie's lawyer, Guy Poupart, argued his client had a limited role in the tragedy. He also played up the testimony of several witnesses who described him as competent and reliable and someone they trusted.

As well, he noted that a good portion of the Crown's case did not involve Labrie.