The NTSB investigation found that the point of derailment was a rail fracture several hundred feet before the bridge. The section of rail, which was examined in the NTSB's materials laboratory in Washington, D.C., showed evidence of rolling contact fatigue, a gradual breakdown of the rail-head surface.
The derailment occurred on CSX's OML Subdivision, which the railroad had documented ultrasonic testing occurring 11 to 12 times a year since 2010, a frequency 12 times what is required.
According to the report, "At the point of derailment, the rail fractured due to a detail fracture that initiated from head checks in the [gauge] corner of the rail head...Material properties did not appear to be a significant factor in the failure. NTSB investigators determined that the defect extended across just 24 percent of the remaining head area, compared to other cases where it extended from 70 to 80 percent of the remaining head area, which indicates the stresses on the rail were relatively high at the time of failure. The high stresses likely resulted from a worn rail head that was approaching levels for scheduled replacement combined with poor ballast conditions and high axle-loads. These conditions produce defects that can grow relatively quickly and can fail at a relatively small size."
As a result of this and earlier accidents investigated by the NTSB, the Federal Railroad Administration established a Rail Failure Working Group to study the effects of railhead wear and resulting rail surface conditions. Its recommendations were adopted by the Rail Safety Advisory Committee in April.
Two people who were sitting on a CSX bridge were killed when coal spilled out of the rail cars during the derailment, which occurred shortly before midnight in downtown Ellicott City. The presence in the rail right-of-way of the two fatally injured persons, did not contribute to the derailment in any way.
Of the 80 cars in the train, the first 21 derailed, with seven falling into a parking lot below the track. The train had been traveling 23 mph, below the speed limit for that section of track.
The NTSB will also hold a public forum next year to explore and educate the public about the dangers associated with unauthorized individuals in the railroad right-of-way.