Freight trains frequently carry hazardous materials through the City of Mississauga. CP train #54 was carrying 106 rail cars from Sarnia to Toronto on a weekly scheduled run. Thirty-eight cars were carrying cargo that the Canadian Transport Commission designated as hazardous materials, including liquid styrene, caustic soda, liquid petroleum products, and liquid chlorine. At approximately 11:53 pm, as the train crossed Burnhamthorpe Road, an axel bearing failed and one rail car lost a pair of wheels. The train continued until, at approximately 11:56 pm at the Mavis Road crossing, 24 rail cars derailed. The explosion, caused by ruptures in butane- and propane-carrying rail cars, was seen more than 100 kilometres away. Several subsequent explosions, one of which hurled a 90-ton tanker car filled with liquid propane more than 675 metres away from the derailment site, followed the first explosion within minutes. The initial cause of the derailment was a “hot box” – or an overheated journal box that connects the moving axle of the wheel to the car above. Car 33 had an older model journal box, which required lubrication by oil to avoid overheating. As the train reached Mississauga, the “hot box” failed. When the dangling undercarriage of the damaged car left the rail tracks, 23 other cars followed it, 19 of which carried dangerous commodities. In the midst of the burning wreckage was a leaking chlorine car. This, coupled with the magnitude of the accident and weather conditions, resulted in a series of expanding evacuations, which by Sunday evening had encompassed nearly all of the City of Mississauga along with parts of the neighbouring towns of Oakville and of Etobicoke. The emergency response saw an evacuation of more than 240,000 residents.

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