Causes
On the 33rd car, 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 who were close to Mississauga thought the train was afire. The friction eventually burned through the axle and bearing, and as the train was passing the Burnhamthorpe Road level crossing, a wheelset (one axle and pair of wheels) fell off completely.
Read more about this topic: 1979 Mississauga Train Derailment