Investigators previously revealed the train entered the curve at 78 miles per hour.
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.
New details released by the the National Transportation Safety Board Thursday revealed more about the last moments on the Amtrak passenger train that derailed on its way to Portland, and the first from the engineer's perspective.
The train - which consisted of a leading and trailing locomotive, a power vehicle, 10 passenger cars and a luggage auto - was traveling at 78 miles per hour when it derailed from a highway overpass near DuPont.
The engineer operating a high-speed Amtrak train when it flew from the tracks during its debut run in December applied the brakes ahead of the deadly derailment, but only after missing key signage that would have prompted him to slow down the locomotive sooner.
The engineer said he then misinterpreted another signal at the 19.8 milepost.
He had been qualified on the Point Defiance Bypass section of the track by completing 7-10 observational trips, as well as three trips where he operated the equipment. But he didn't see a mile marker ahead of the curve or a sign warning in advance of the speed-limit drop, the NTSB said. "He had never worked with the engineer before".
They did not speak much during the train ride and the conductor spent his time familiarizing himself with the route, he told investigators. The conductor said he heard the engineer mumble something, and when he looked up, he sensed the train becoming airborne.
The conductor who was also in the locomotive at the time of the derailment has been with Amtrak since 2010 and was promoted to his current post in 2011.
The NTSB said it was considering human performance, signals, train control, the tracks and engineering in its investigation, which is expected to last one to two years.
Investigators said they will compare the men's accounts of what happened with video from cameras placed in the locomotives and information from the data recorder.
The second man in the locomotive, training to qualify for the route, told investigators that he was studying paperwork just before the crash.