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MONTREAL (Reuters) - A Canadian judge on Tuesday told jurors to continue deliberations, despite their difficulty in reaching a verdict in the joint trial of three former rail employees in the 2013 Lac Megantic train derailment that killed 47 in Quebec, local media said.

The jury had earlier told the judge it was at an impasse, said Charles Shearson, a lawyer for one of the three defendants.

In July 2013, the downtown section of the town of Lac Megantic was destroyed following the derailment of a Montreal Maine & Atlantic Railway Ltd train carrying crude oil.

Locomotive engineer Tom Harding, 56, operations manager Jean Demaître, 53, and rail traffic controller Richard Labrie, 59, face charges of criminal negligence causing death in the crude-by-rail disaster, according to court documents.

The three worked for the now-defunct Montreal Maine & Atlantic railway which operated the runaway train that was carrying 7.7 million liters of volatile Bakken crude oil, according to a 2014 accident report.

The derailment sparked calls to improve rail safety in North America, with Canada ending the use of one-man crews to move dangerous goods and enhancing protection standards for tank cars used to transport crude by rail.

The trial comes amid an expected resurgence in rail shipments of less volatile Canadian crude in 2018 as tight pipeline capacity is pushing more oil onto railroads.

The jury, which has been in deliberations for six days, had previously asked Quebec Superior Court Judge Gaetan Dumas for a dictionary and to clarify judicial matters, like the concept of “reasonable doubt,” local media reported. Their request for the dictionary was refused.

All three pled not guilty and did not testify during the trial in Sherbrooke, Quebec.