On November 10, 1979, a freight train left its tracks on the Canadian Pacific rail line while carrying explosive and poisonous chemicals just north of the intersection of Mavis Road and Dundas Street. One of the tank cars carrying propane exploded. Since other tank cars were carrying chlorine, the decision was made to evacuate nearby residents. With the possibility of a deadly cloud of chlorine gas spreading through Mississauga, 218,000 people were evacuated. Residents were allowed to return home, once the area was safe. It was one of the largest evacuations in North American history.