Dec 11 (Reuters) - A Canadian Pacific Railway Ltd train carrying crude and other cargo derailed in April 2013 because of a broken wheel, which Canada's transport safety watchdog said could have been replaced but was not due to differing guidelines on when repairs should be made.
Four days before the incident the wheel exceeded the American Association of Railroads' (AAR) removal threshold in a trackside test, but under CP's higher threshold, which is similar to that of other major railways, it remained in service, said the Transportation Safety Board (TSP).
After the test, "... CP could have set the car out immediately, replaced the No. 1 wheel set and charged the car owner for the work in accordance with AAR rules," said the TSB in its report, released on Thursday.
Regulations do not force railways to replace wheels based on trackside tests in Canada or the United States, an issue the Canadian watchdog raised in a 2011 advisory. Transport Canada promised a "comprehensive review" but on Thursday the TSB said "to date, there have been no tangible developments."
The TSB said AAR rules are based on technical studies done in the early 1990s. It said CP's thresholds were "established primarily by industry practice and in order to manage the volume of wheels removed," not an engineering analysis.
Wheel Impact Load Detectors, installed along train tracks, can catch problems with wheels before they fail, but railways decide which readings merit a repair.
Under AAR rules, wheels are to be removed if a detector registers an impact of 90,000 pounds or greater. Members must accept equipment that meets AAR standards, but can take other equipment at their discretion.
In its report, the TSB said CP does not necessarily remove wheels unless they measure above 130,000 pounds, which is typical of the industry. Under CN Rail's guidelines, trains are slowed and suspect cars removed above 150,000 pounds.
On March 30, 2013, the wheel that would later fail registered an impact of 103,900 pounds, the TSB said. It met AAR removal criteria five other times between December 2012 and the accident.
While the vast majority of freight trains reach their destination without incident, crude oil shipments have put the industry under new pressure to prevent derailments.
CP did not immediately respond to a request for comment. (Reporting by Allison Martell; Editing by Bernard Orr)