 Most of the derailments occurred on trains with two SDP40Fs on the front and at least one trailing baggage car.While the rear truck of the second locomotive and the front truck of the baggage car were pinpointed as the source of the derailment, the actual cause of the derailments was unclear.EMD, Amtrak, the Association of American Railroads (AAR), and the FRA tested the locomotive thoroughly, with suspicion falling on the "hollow bolster" truck design.