A more stringent inspection of old tank car axles might have prevented an explosive BNSF train collision in Casselton, North Dakota, in December 2013, according to documents posted by the National Transportation Safety Board. In a letter to the Association of American Railroads (AAR) dated April 7, 2014, the NTSB said tests carried out by BNSF on its train car axles might have missed internal faults that more thorough examinations would have detected. The NTSB recommends that railroads use ultrasonic tests for all second hand axles. The letter was originally posted in an NTSB database last year, but was republished on Monday as part of a package of documents relating to an 15-month investigation into the accident. The incident occurred when a train carrying crude oil collided with a derailed BNSF grain train, causing a huge explosion. No one was hurt in the blast. BNSF did carry out the required tests in April 2010. However, had the axles undergone ultrasonic testing "the internal material defect would likely have been found and the axle would not have been allowed to return to service," the NTSB said. The NTSB recommended that testing on second hand axles should be "specifically designed to locate internal material defects in axles." The safety recommendation is the only one to come from the ongoing NTSB investigation into the accident, which stoked a debate about the safety of transporting oil by rail. Berkshire Hathaway-owned BNSF was not immediately able to provide a response. While the NTSB is yet to draw any conclusions from its investigation, the letter shows the federal body considers the faulty axel on the derailed grain train to be a likely cause of the accident. Since the accident's immediate aftermath, federal investigators have focused on a broken axle found at the scene. A few weeks after the accident, the AAR advised its members that an identical model of axel was involved in two recent derailments and urged them to take them out of service and return them to the manufacturer, Pennsylvania-based Standard Steel[TRMRNS.UL], according to other documents posted Monday. There were 43 such axles believed to be in service, the AAR said. Once returned, the axles were to be tested and the results provided to the AAR. (Reuters)