On 6 January 2018, a Canadian National Railway (CN) freight train, proceeding westward at about 50 mph on the Redditt Subdivision, experienced a train-initiated emergency brake application. An inspection revealed that 23 cars had derailed at Mile 166.33, including eight cars carrying dangerous goods. There were no injuries, and no product was released.
The investigation found that the derailment occurred when the R4 wheel on the 38th car from the head end failed progressively as a result of a vertical split rim fracture. This fracture had propagated circumferentially in opposite directions from the point of origin, reaching a length of 68 inches. The wheel tread and flange were damaged, and a section of the wheel tread and plate had broken away.
In the two days preceding the derailment, the same wheel had recorded five wheel impacts that exceeded the Association of American Railroads (AAR) Rule 41 condemning criterion of 90 kips. However, CN’s guidelines for wheel impact load detectors permitted the car to continue to the next certified car inspection location. The wheel remained in service and failed about four and a half hours after recording a peak impact of 109 kips at a wheel impact load detector site located at Mile 10.9 of the Redditt Subdivision.
The investigation highlighted two risk factors. Without implementation of additional enhancements for cracked-wheel detection to augment wheel impact load detector technology, there is a continued risk that a wheel with an emerging vertical split rim defect will not be identified and removed before it fails. Additionally, if axial ultrasonic testing is not performed on a wheel rim face following wheel reprofiling at railway shops, wheels with vertical split rim defects may return to service, thus increasing the risk of a derailment.
From 2013 to 2018, in Canada, CN removed 358 485 wheel sets (an average of 60 000 per year) under its guidelines as well as those of the AAR Rule 41. Despite significant improvements in detection and inspection, wheels continue to fail in service, sometimes resulting in derailments. The industry is researching additional technologies to help detect emerging sub-surface cracks in wheels. These technologies include automated cracked-wheel detection system, WILDCaRD system, and wheel impact trending, among others.
See the investigation page