A written directive attached to the letter, obtained by The Canadian Press, raises concerns over the company failing to identify and assess hazards to employees resulting from increased switching in the rail yard.

The union says negotiations with the railway will resume immediately in an attempt to achieve an agreement and avoid a work stoppage.

A Coalhurst, Alta. fire official told Global News a train derailment that occurred in the town on Friday saw six cars carrying crude oil go off the tracks.