The Transportation Safety Board says a seized axle and crew fatigue were behind a train derailment between Terrace and Prince Rupert.
A westbound CN train with two locomotives and 153 loaded intermodal cars ran into trouble near Kwinitsa on November 15, 2014.
Crews found that the trailing locomotive and eight intermodal cars with had left the tracks shortly after the train had pulled away from a siding.
TSB investigators determined that the No. 4 axle on the locomotive had locked when overheating parts seized together while the train was stopped at the siding.
The wheels became deformed as they slid along with rail and then came off the tracks as the axle passed through the switch near Kwinitsa.
There were no injuries and no dangerous goods were involved.
The TSB says a wayside inspection system between the siding and the derailment site only examined nine of the 424 axles because the damaged will dislodged sensors on the system. The report notes that the crew did not notice that the scan results were announced much earlier than normal.
Investigators found that the crew members were fatigued at the time of the derailment because shift patterns with variable start and end times left them with erratic sleep patterns.
The TSB probe notes that highly variable shift times may result in a lack of good quality sleep which increases the risk of fatigue-caused accidents.
Since the incident, CN has reformatted its wayside inspection systems to include axle counts as part of the post-scan announcement.