The 1979 Mississauga Train Derailment

Just before midnight on Saturday, November 10, 1979, a 106-car Canadian Pacific freight train travelling from Windsor to Toronto derailed near Mavis Road in Mississauga. What followed became one of the largest peacetime evacuations in North American history — and, remarkably, killed no one.

A hot box and a chlorine tank

The generally cited cause was mechanical: an overheated, improperly lubricated wheel bearing caused an axle to fail, sending roughly two dozen cars off the tracks. Several of the derailed tank cars carried propane, which exploded and burned in a fireball visible for kilometres. Far more dangerous was a tank car carrying an estimated 90 tonnes of liquid chlorine, along with other cars of caustic soda, styrene and toluene — a ruptured or venting chlorine car threatened to release a toxic gas cloud capable of severe lung damage if it drifted into populated neighbourhoods.

The largest evacuation of its era

Fearing exactly that, authorities ordered an evacuation that eventually covered most of Mississauga, then a city of roughly 280,000 people. Figures commonly cited in retrospectives put the number evacuated at approximately 240,000 — at the time, the largest peacetime evacuation in North American history, a record later surpassed by the 2005 Hurricane Katrina evacuations. The evacuation ran for close to a week, from November 10 to around November 16, and was widely noted for its orderliness, carried out with minimal panic or looting under the direction of Mississauga's mayor at the time, Hazel McCallion.

Why it's called a miracle

Despite explosions, a chemical fire that burned for days, and a toxic gas threat over a major suburb, there were no deaths and no serious injuries directly attributed to the derailment — a combination of fortunate timing (the crash happened late at night near few people), a fast, coordinated evacuation, and weather conditions that kept the worst of the chlorine plume away from dense residential areas. The derailment prompted lasting changes to Canadian rail-safety and hazardous-materials regulations, and it's still commemorated locally as the "Mississauga Miracle."

Sources

• CBC News — Mississauga Miracle: Remembering the Disaster That Forced 240,000 People to Flee
• City of Mississauga — Commemorating the 40th Anniversary of the Mississauga Miracle