But Earl Weener said it was unclear whether the carnage was the result of human error or mechanical trouble.
Safety experts said the tragedy might have been prevented if Metro-North Railroad had installed automated crash-avoidance technology that safety authorities have been urging for decades.
The locomotive’s speed was extracted from the train’s two data recorders after the accident on Sunday morning, which happened in the Bronx along a bend so sharp that the speed limit drops from 70 to 30mph.
Mr Weener, of the National Transportation Safety Board, would not disclose what William Rockefeller, the engineer operating the train, told investigators and he said results of drug and alcohol tests were not yet available.
Investigators are also examining the engineer’s mobile phone, apparently to determine whether he was distracted.
Mr Rockefeller, 46, was injured and “is totally traumatised by everything that has happened,” said Anthony Bottalico, executive director of the rail workers’ union. He said Mr Rockefeller was co-operating fully with investigators.
“He’s a sincere human being with an impeccable record that I know of. He’s diligent and competent,” Mr Bottalico said. Mr Rockefeller has been an engineer for about 11 years and a Metro-North employee for about 20, he said.
Mr Weener sketched a scenario that suggested that the throttle was let up and the brakes fully applied far too late to stave off disaster.
He said the throttle went to idle six seconds before the derailed train came to a complete stop – “very late in the game” for a train going that fast – and the brakes were fully engaged five seconds before the train stopped.
It takes about a mile for a train going 70mph to stop, according to Steve Ditmeyer, a former Federal Railroad Administration official who now teaches at Michigan State University.
Asked whether the tragedy was the result of human error or faulty brakes, Mr Weener said: “The answer is, at this point in time, we can’t tell.”