The seven-car train was traveling at 82 miles per hour (132 km per hour), nearly three times the speed limit for the curved section of track where it crashed, investigators have said.
The driver, William Rockefeller, 46, applied the brakes five seconds before it derailed. The crash also critically injured 11 people and snarled travel for the roughly 26,000 regular commuters on the Metro-North Hudson line that serves suburbs north of New York City.
On Tuesday, Rockefeller told National Transportation Safety Board investigators that "he nodded. He zoned out," Anthony Bottalico, the general chairman of the driver's labor union, the Association of Commuter Rail Employees, told Reuters.
Rockefeller told investigators that "by the time he realized (what was happening) it was almost into the curve," Bottalico said. "He put the train into neutral and put the brakes on immediately. That's what he acknowledged he did."
The NTSB has cautioned that its investigation would continue for weeks, if not months, and it was far from reaching a conclusion on the cause. Alcohol tests on Rockefeller came back negative, NTSB member Earl Weener told a news conference on Tuesday, adding the results of drug tests were still pending.
The train might have benefited from a Positive Train Control (PTC) system to stop or slow a speeding train, Weener said.
"For more than 20 years, the NTSB has recommended implementation" of PTC, Weener said. "Since this is a derailment, it's possible that PTC could have prevented it."
Railroad experts have been advocating for PTC systems for years, but they are expensive and complicated and often incompatible for all trains within a single transit system.
The Metropolitan Transportation Authority, which runs Metro-North, said it began work to install Positive Train Control in 2009 with a goal of implementing it by 2015. The authority said it has budgeted nearly $600 million, with at least another $300 million needed, and even then was unlikely to meet the 2015 deadline.
"I was in a daze"
Apart from the equipment and technology, investigators are looking at Rockefeller, a volunteer firefighter who was married two years ago. The NTSB, which interviewed Rockefeller on Tuesday afternoon, said it is focusing on all his activities in the 72 hours before the crash, Weener said.
Rockefeller spent about two hours with NTSB investigators, said Bottalico of the driver's labor union. The NTSB said in a statement late on Tuesday evening, after Bottalico's news interviews, that it had removed the Association of Commuter Rail Employees from the investigation, citing a breach of confidential information.
Rockefeller had never been disciplined for job performance in his 10 years as a train driver, his union said. Rockefeller has retained a defense lawyer, Jeffrey Chartier, who did not respond to a request for comment.
One source involved in the ongoing investigation quoted Rockefeller directly as having told investigators, "I was in a daze" in the moments before the crash.
Asked whether Rockefeller dozed off, the source said, "It's more like a highway hypnosis. You're looking straight ahead and you're seeing rail and rail and rail, and you lose perspective."
A similar condition of temporary lapse of consciousness, known as microsleep, was blamed for a 2008 Boston light rail crash that killed the operator.
In 2003, assistant captain Richard Smith blacked out at the wheel of the Staten Island ferry and crashed on docking, killing 11 people. Smith pleaded guilty to negligent homicide and was sentenced to 18 months in prison.
Rockefeller could not fully recall what happened, except that at some point he suddenly came out of the temporary daze, realized the train was going too fast and into a dangerous curve, and applied the brakes, said the law-enforcement source, who has access to official reports on the investigation and requested anonymity.
"Look, it doesn't make what happened any better or anything, but this is comparable to driving a car and looking at the white lines, and sort of nodding off for a minute," Bottalico said.
If criminal charges are warranted, they would be brought by Bronx District Attorney Robert Johnson, a spokesman for Johnson said. "We think when final record is looked at, they'll realize there was no criminal intent here," Bottalico said. "Speed was high, but it was an accident."