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The Mississauga train derailment of 1979 occurred on November 10, 1979, when a 106-car Canadian Pacific freight train carrying explosive and poisonous chemicals from Windsor, Ontario was derailed near the intersection of Mavis Road and Dundas Street in Mississauga, Ontario. As a result of the derailment, over 200,000 people were evacuated in what was then the largest peacetime evacuation in North America up until the Hurricane Katrina evacuation in New Orleans in 2005. Fortunately and remarkably, there were no deaths resulting from the spill.

On the 33rd car, in the middle of the train's load, heat began to build up in an improperly-lubricated journal bearing on one of the wheels, resulting in the condition known among train workers as a hot box. Residents living beside the tracks reported smoke and sparks coming from the car, and those close to Mississauga thought the train was on fire. The friction eventually burned through the axle and bearing, and as the train was passing the Burnhamthorpe Road level crossing, one axle and pair of wheels fell completely off.

At 11:53 p.m., at the Mavis Road crossing, the damaged undercarriage left the track, causing most of the rest of the train to derail. The impact caused several tanker cars filled with propane to burst into flames.

The derailment also ruptured several other tankers, spilling styrene, toluene, propane, caustic soda, and chlorine onto the tracks and into the air. A huge explosion resulted, sending a fireball 1,500 m into the sky which could be seen from 100 km away. As the flames were erupting, a train worker managed to close a brake valve on the undamaged 32nd car, allowing the engineer to drive the front part of the train eastward along the tracks and out of danger.

After further explosions, firefighters concentrated on cooling cars, allowing the fire to burn itself out, but a ruptured chlorine tank became a cause for concern. With the possibility of a deadly cloud of chlorine gas spreading through suburban Mississauga, over 200,000 people were evacuated and a considerable number of residents who were not evacuated (mostly the extreme west and north of Mississauga) allowed evacuated residents to stay with them until the crisis abated. Some of these evacuees were later evacuated twice as their hosts were later evacutated also. The evacuation was overseen by the mayor of Mississauga, Hazel McCallion, along with the Peel Regional Police and other governmental authorities. McCallion sprained her ankle early in the crisis, but continued to hobble to press conferences and updates.

Within a few days Mississauga was practically a ghost town, until the contamination had been cleared, the danger neutralized and residents were allowed to return to their homes. The city was finally reopened in the evening of November 16. The chlorine tank was emptied on November 19.

At the time, it was the largest peacetime evacuation in North American history, and is currently the second largest after the evacuation of New Orleans, Louisiana after the impact of Hurricane Katrina.

Due to the speed and efficiency with which it was conducted, many cities later studied and modeled their own emergency plans after Mississauga's. For years afterwards, the name "Mississauga" was for many Canadians associated with this major rail disaster.