A Quebec Superior Court judge has urged the jury in the trial of three former railway workers charged in the Lac-Mégantic rail disaster to continue deliberations and try to reach a unanimous verdict.

"Would you please try once again to reach a verdict?" Justice Gaétan Dumas asked the eight men and four women who have been deliberating for six days now at the courthouse in Sherbrooke, Que.

On Tuesday afternoon, jurors sent a note to the judge seeking guidance on how to proceed.

"We are at an impasse. What happens in the event we cannot agree?" jurors asked.

The jurors are deciding the fate of Tom Harding, Richard Labrie and Jean Demaître. The three men are each charged with criminal negligence causing death, in the July 6, 2013, tragedy in which 47 people were killed when a runaway train carrying crude oil derailed and exploded.

All three men have pleaded not guilty.

Harding was the train's engineer, Labrie the rail traffic controller, and Demaître was the manager of train operations in Quebec.

Called back to the courtroom, the jurors looked sombre and disappointed once the judge asked them to continue deliberations.

Dumas told them it is not mandatory that they reach a unanimous verdict, but it's the desirable outcome.

"I have the authority to discharge you from giving a verdict where further deliberation is evidently useless in helping you reach a unanimous verdict, however, that right is not to be given lightly or at the first sign of difficulty," he said.

Dumas told them juries are often able to reach a verdict when given more time.

He also reminded them they took an oath promising to render a verdict based on their opinion of the evidence.

"We expect you to pool your views of the evidence, but it does not require you to put aside your own views of the evidence," he said.

"You should not be tempted to change your mind in order to reach a unanimous verdict."

"Failure to reach a unanimous verdict will not reflect badly on you," Dumas added, "provided you have tried to the best of your abilities, based on the evidence."

Prior to calling the jurors back to the courtroom to give them guidance, Dumas told lawyers, reporters and the courtroom audience that it's possible the jury has agreed on a verdict for one or two of the accused but not for the third.

"I don't see any legal problems if there is a unanimous verdict for one and an impasse for the other two," he said.

"If, after a reasonable delay, they come back to us and are still at an impasse, I could remind them that there are three separate trials and it's possible for them to reach verdicts on just one or two," Dumas told the courtroom.

Outside the courtroom, lawyers for the three men said they were reluctant to speculate on the jury's deliberations.

"There are nine possible verdicts," said Charles Shearson, one of the lawyers for Harding, the locomotive engineer.

Each of the three defendants could be found guilty or not guilty of criminal negligence causing 47 deaths. A guilty verdict carries a maximum sentence of life in prison.

However, in his instructions to jurors last Wednesday, Dumas said that if they find Harding not guilty of criminal negligence, they must consider the lesser charges of dangerous operation of railway equipment causing death, and dangerous operation of railway equipment. Those charges carry maximum sentences of 14 years and five years, respectively.