A complex combination of six factors led to a major train derailment in Strathroy in July of 2017.
An investigation by the Transportation Safety Board of Canada (TSB) found the problem originated with an older car loaded with cement.
A wheel on that car, the 109th in the 128-car train, climbed the rail leading to the derailment of 13 cars near the Metcalfe Street West crossing on July 19, 2017.
TSB senior investigator Rob Bruder said in over 40 years, he's never seen such a complex derailment involving so many variables, and everything was within safety standards.
"The car was very safe to travel without any of the other factors," said Bruder. "So if you take a look at that factor, the age of the car and that it's somewhat vulnerable to rock and roll and then you take a look at the track conditions themselves, there's nothing condemnable there either, but there's a bit of a low spot through the crossing at the point of derailment."
The TSB said one of the derailed cars had last contained liquefied petroleum gas, but there was no release of dangerous goods and there were no injuries.
Bruder said the locomotive engineer and conductor handled the train very well by "applying a controlled brake application."
"He did not dump it into emergency, which would have put the train likely all over the place much earlier and potentially in much more vulnerable parts of the town," he said.
Bruder said since the derailment, CN has purchased specialized tools and equipment to measure very technical aspects of older cars.