The passenger train carrying 83 passengers and crew was traveling almost 80 miles an hour when it went off the tracks on a curve where the speed limit was 30 miles an hour, landing on an interstate below.
The safety board said Thursday that the 55-year-old engineer, who was interviewed last week after suffering serious injuries in the crash, told the agency the train was traveling at about 80 miles per hour as it passed milepost 15.5 on its inaugural journey from Seattle to Portland, Oregon.
"The engineer said that he did see the wayside signal at milepost 19.8 (at the accident curve) but mistook it for another signal, which was north of the curve", the NTSB wrote in its report.
He told the investigators he recalled seeing mileposts 16 and 17 but did not remember seeing the sign for milepost 18.
The engineer completed seven to 10 "observational trips" in a locomotive on the new stretch of track in the five weeks prior to the derailment, the NTSB said.
The fatal trip marked the second time the engineer had officially operated it going southbound on the route.
In a news release, the NTSB said the engineer knew there would be a curve with a 30 miles per hour speed limit at milepost 19.8, but he didn't remember seeing milepost 18 or the 30 miles per hour advance speed sign.
The two had never worked together before.
The engineer told investigators that he didn't consider having a qualifying conductor in the cab to be a distraction. He told investigators that the engineer appeared alert during a job briefing and while operating the train.
Shortly before the derailment, the conductor heard the engineer mumble something, looked up and felt the train go "airborne", according to the report.
The conductor of the train was a 48-year-old male, hired by Amtrak in 2010 as an assistant conductor and promoted to conductor in 2011.
Garrick Freeman was identified as the conductor after filing a lawsuit against Amtrak, claiming the company failed to provide a safe work environment.
The investigation is expected to last anywhere from 1-2 years.
In addition to human performance and operations, investigators are developing information on a wide range of areas, including signals and train control, track and engineering, mechanical, crashworthiness, survival factors and recorders.