How do they know that the train was a runaway ? I'll wait here.
They found the five locomotives with their brakes still set. It isn't clear to me if they found them where it had been parked 4km uphill, or somewhere intermediate. But the locomotives did not go down into town with the tank cars.
UPDATE: this from the CBC this afternoon --
"It is still not clear what caused the train cars to roll downhill and leave the tracks. The company reports that the train was stopped 6.8 kilometres uphill from the town at 11:25 p.m. by the engineer. At some point before 1:15 a.m. ET, the train moved downhill. Railway personnel pulled 13 cars intact from the rear of the train at the scene.“We will be taking a look at the black box. Hopefully we’ll recover that,” Glen Pilon, an investigator with the Transportation Safety Board of Canada, said Saturday afternoon."
I'm really not at all clear about what happened now.