Union leader Anthony Bottalico said he was confident the investigation would reveal there was no criminal intent.
"At this point in time, we can't tell" whether the answer is faulty brakes or a human mistake, Weener said.
Investigators began talking to the train's engineer Monday but were unlikely to continue the interview until Wednesday, National Transportation Safety Board spokesman Keith Holloway said Tuesday. He said he had no information on the reason; Bottalico said it was because William Rockefeller hadn't slept in almost 24 hours and was "very distraught."
Bottalico said the engineer planned to have a lawyer accompany him to the interview. The attorney didn't immediately return a call Tuesday.
The New York Police Department is conducting its own investigation with assistance from the Bronx district attorney's office in the event the derailment becomes a criminal case.
Weener sketched a scenario suggesting that the throttle was let up and the brakes were fully applied way too late to stave off the crash. 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 quarter-mile to a half-mile to stop a train going 82 mph, according to Kevin Thompson, a Federal Railroad Administration spokesman.
Investigators are not aware of any problems with the brakes during the nine stops the train made before the derailment, Weener said.
Weener would not disclose what investigators know about the engineer's version of events, and he said the results of drug and alcohol tests were not yet available. Investigators are also examining the engineer's cellphone; engineers are allowed to carry cellphones but prohibited from using them during a train's run.