New Delhi, July 17 -- TORONTO - Three freight trains halted north of Thunder Bay as wildfire flames closed in on the rail lines last week, their crews watching through cab windows as the forest burned on every side, the horizon a deepening orange. One of the trains was carrying combustible cargo. All crew members were safely evacuated before the fire could reach them, and no injuries were reported. "No freight is worth risking workers' lives," the head of the railway union, Francois Laporte, said afterward.

Ontario's wildfire season, already the most destructive in years, crossed into a different kind of emergency on Thursday. The fires burning through the province's remote boreal north for weeks produced a plume of smoke so large and so...