Investigation of a train de-railment near Nipigon in January 2015 has revealed it was caused by a crack in the rail frame.
The train carrying propane cars derailed after the railway couldn't hold the 11th car, causing the following 21 cars to crash with it.
The report indicates the already formed crack was at a bolt hole, and wasn't detected during regular inspection as it was behind the joint bars, making detection difficult.
Snow and cold temperatures at the time of the crash made things worse.
Only one car lost all its product, and another one partially.
One crew member had to be treated for minor injuries after breathing in the propane.