In its final report on the accident, the NTSB said it found evidence that the section of CSX Transportation rail showed signs of "gradual deterioration of the rail-head surface" from passing trains.
The finding makes official preliminary conclusions about a rail break contributing to the accident that were released in a docket of investigatory documents last month.
The break in the rail was several hundred feet from where 19-year-old college students Rose Louese Mayr and Elizabeth Conway Nass, both graduates of Mount Hebron High School, were seated on an overpass that carries the railroad above Main Street.
Mayr and Nass were trespassing at the time of the incident, as the bridge is part of the railroad's right-of-way. The NTSB said their presence next to the tracks "placed them in harm's way" but "did not contribute to the derailment in any way."
The two women's parents have said they are considering litigation against CSX unless the railroad offers a public apology for the incident and a financial settlement.
Ronald Goldman, the parents' attorney, said Thursday that the families were "deeply disappointed" with the NTSB report, in part because it "fails to go further" in making recommendations that would force real change in the railroad industry.
"We have had investigation after investigation of defective track that leads to derailments, but nobody's doing the hard work that I think needs to be done to determine how they can prevent track from becoming defective in the first place," Goldman said. "They should do investigations and studies to determine if rails should be life-limited. We haven't done that hard work. They do it in aviation, but not in rail."
In a statement, CSX largely stuck to comments it has made before: that its sympathies remain with the families, that it values and is committed to safety, and that it follows all federal regulations and its own internal policies.
"We are evaluating the conclusions and look forward to applying lessons learned through the investigation to further enhance our ability to prevent such incidents in the future," the company said of the NTSB investigation.
The NTSB report found that CSX had been conducting routine inspections of local tracks, including ultrasonic testing more frequently than is required by federal regulations, in part because of a "history of rail defects" in the area and an "increase in tonnage due to a rise in coal traffic over the previous years."
The report also found stress on the tracks was "relatively high" at the time of the 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," the NTSB found. "These conditions produce defects that can grow relatively quickly and can fail at a relatively small size."
The last ultrasonic test for internal rail flaws in the area occurred Aug. 3, 2012, 17 days before the accident, the NTSB investigation found, but "no defective rails were marked near the derailment area." Defects were noted along other sections of the more than 15 miles of track studied.
The derailment sent 21 train cars off the tracks, seven of which landed in a nearby parking lot. Damage was estimated at $1.9 million. Mayr and Nass were asphyxiated after being buried in coal from an overturned car on the overpass.
According to the NTSB report, the pair had climbed over "a short wooden fence" sometime before midnight on the cool summer night to enter the restricted area. In line with local speculation at the time, the report also notes that Mayr and Nass were "seen consuming alcoholic beverages prior to the accident."
At the time of their autopsies by the medical examiner, the report said, the two women had ethanol in the vitreous fluid of their eyes equivalent to blood- alcohol levels of 0.05 percent and 0.03 percent. The report did not ascribe the levels to the women individually. At those levels, the women would have been able to drive were they of legal age to drink.
Goldman said it was "offensive" to mention alcohol in the report at all, calling it a "deflection of responsibility away from CSX" that would cause the families more distress.
"It had nothing to do whatsoever with this accident. They were not under the influence of alcohol in any legal sense of the word. They were sitting quietly," he said. "Many, many, many people routinely go to that area because it's beautiful, it's scenic, it's part of the town, part of the culture."
Eric Weiss, an NTSB spokesman, said the report was a "thorough, factual" accounting of the derailment, which he called a "tragedy." The agency noted in the report that CSX installed chain-link fencing along its right-of-way in the area after the derailment, where no fencing had existed before.