In its investigation report, the Transportation Safety Board of Canada (TSB) identified a number of factors that led to a broken rail and derailment of a Canadian National (CN) train near Saskatoon, Saskatchewan in January 2019.
Article continues below
On 22 January 2019, a CN freight train was proceeding southward at 31 mph on the Warman Subdivision in Saskatchewan. At 09:23 local time, the train experienced a rough ride as it crossed over the median between divided Highway 11. A minute later, a train-initiated emergency brake application occurred as 29 loaded grain cars and a mid-train locomotive derailed.
Some of the derailed grain cars released their loads, and the derailed locomotive caught fire, but was quickly extinguished. No dangerous goods were involved and there were no injuries.
The investigation found that the rail head was missing at a joint between two pieces of rail, leaving a gap in the rail surface. As the head-end of the train travelled over the gap in the rail, the impact from the wheels further damaged the rail until it failed under the 27th car, resulting in the derailment. Analysis determined that the fracture that broke the rail likely initiated at a bolt hole.
However, no cracks had been detected when the track was inspected ultrasonically less than a month earlier, suggesting that the crack had progressed from a non-detectable state to rail failure within weeks. Trains travelling through this section of track passed from well-supported track in the highway roadways onto less stiff track in the median between the roadways, subjecting the rail to bending forces. Track cross-level and surface alignment progressively deteriorated, in turn increasing the deflection of the rail and the eventual failure.
It was also found that a damaged segment of track in the highway median had been recently repaired using rail manufactured in 1953. While the rail had been ultrasonically inspected before being placed in the track, manufacturing processes at the time the rail was made resulted in it having lower fracture toughness, lessening its resistance to the propagation of fractures. A bolt hole crack in the 1953 rail developed and quickly propagated, resulting in the broken rail.
The investigation revealed the risks associated with using rail manufactured with lower fracture toughness to repair sections of track that experience frequent bending forces. In such a case, there is a risk that rail defects will develop and progress to failure faster than they can be detected by the rail flaw inspection regime.
Prior to the occurrence, the Warman Subdivision had seen an increase in traffic volume, exceeding a regulatory threshold thereby requiring more frequent inspections; however, the frequency of inspection had not been increased.
The investigation found that if in-year traffic volumes increase beyond the threshold, and the frequency of inspections is established only during the subsequent annual traffic volume review, developing safety defects in the track may not be detected, increasing the risk of failure and accidents.
It was also determined that although the CN track information system recorded details of repairs made to the track, it did not provide sufficient resolution to assess the work conducted at individual joints where short pieces of rail were installed close together, as was the case in this occurrence. ■