A train exhaust spark is believed to have been what ignited five railcars being pulled through London last month.
The Transportation Safety Board of Canada (TSB) released the finding Wednesday after gathering information and evaluating evidence in the April 21 incident.
"The cause of the fire was likely sparks from the locomotive exhaust along with the airflow of the moving train causing the fire to spread rearward to each subsequent car," said TSB spokesperson Hugo Fontaine.
He added that the train crew became aware of the burning railcars somewhere between Strathroy-Caradoc and London.
"They immediately stopped and cut away from the tail end of their train, isolating the six open top gondola cars," said Fontaine. "They then slowly pulled the cars to a safe location just east of the Waterloo Street crossing [in downtown London] adjacent to a commercial fire hydrant. The crew then separated the locomotives from the gondola cars."
More than two dozen London firefighters met the train at that location and worked quickly to extinguish the blaze with water and retardant foam. The charred railcars were then moved to a nearby railyard.
"The fire department indicated the fire was fully contained, and there was nothing within the nearby vicinity or right of way that was combustible, therefore there was little to no risk to the public," Fontaine added.
The railcars were transporting used wooden railway ties. The fire caused an estimated $25,000 damage to the railcars and $10,000 damage to a nearby building at the Waterloo Street rail crossing. Video of the burning railcars moving through the city quickly spread on social media and attracted international attention.
The TSB classified the incident as a Class 5 investigation. That particular type of occurrence is not subject to comprehensive investigations followed by an investigation report. Instead, the information on the incident is recorded for possible future safety analysis, statistical reporting, or archival purposes.
Canadian Pacific Railway continues to investigate the fire.