Rail 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 Sunday morning accident, which happened in the Bronx along a bend so sharp that the speed limit drops from 70 mph to 30 mph.
Asked why the train was going so fast, National Transportation Safety Board member Earl Weener said: "That's the question we need to answer."
Weener would not disclose what 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 cellphone, apparently to determine whether he was distracted.
"When I heard about the speed, I gulped," said Sen. Charles Schumer, D-N.Y.
Engineers may not use cellphones while on the train, according to the Metropolitan Transportation Authority, which runs Metro-North.
The engineer, William Rockefeller, was injured and "is totally traumatized by everything that has happened," said Anthony Bottalico, executive director of the rail employees union.
He said Rockefeller, 46, was cooperating fully with investigators.
"He's a sincere human being with an impeccable record that I know of. He's diligent and competent," Bottalico said. Rockefeller has been an engineer for about 11 years and a Metro-North employee for about 20, he said.
Outside Rockefeller's modest house in Germantown, N.Y., police told reporters that at the request of the family, anyone who trespassed would be arrested. Calls to the home went unanswered.
The NTSB's Weener sketched a scenario that suggested that the train's throttle was let up and the brakes were fully applied way 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 quarter-mile to a half-mile to stop a train going 82 mph, Kevin Thompson, Federal Railroad Administration spokesman.
Asked whether the tragedy was the result of human error or faulty brakes, Weener said: "The answer is, at this point in time, we can't tell."
But he said investigators are not aware of any problems with the brakes during the nine stops the train made before the derailment.
The wreck came two years before the federal government's deadline for Metro-North and other railroads to install automatic-slowdown technology designed to prevent catastrophes caused by human error.
Metro-North's parent agency and other railroads have pressed the government to extend Congress' 2015 deadline a few years because of the cost and complexity of the Positive Train Control system, which uses GPS, wireless radio and computers to monitor trains and stop them from colliding, derailing or going the wrong way.
Steve Ditmeyer, a former FRA official who teaches at Michigan State University, said the technology would have monitored the brakes and would not have allowed the train in Sunday's tragedy to exceed the speed limit.
"A properly installed PTC system would have prevented this train from crashing," he said. "If the engineer would not have taken control of slowing the train down, the PTC system would have."
On Sunday, the train was about half full, with about 150 people aboard, when it ran off the rails around 7:20 a.m. while rounding a bend where the Harlem and Hudson rivers meet. The lead car landed inches from the water. In addition to the four people killed, more than 60 were injured.
Seven victims were still in intensive care at one hospital, and two patients were reported in critical condition at another.
The injured included five police officers who were heading to work, according to Mayor Michael Bloomberg.
Gov. Andrew Cuomo said on NBC's "Today" show that he thinks speed will turn out to be a factor in a crash he called "your worst nightmare."
The train was configured with its locomotive pushing from the back instead of pulling at the front. Weener said that is common, and a train's brakes work the same way no matter where the locomotive is. Ditmeyer said the locomotive's location has virtually no effect on train safety.
The dead were identified as Donna L. Smith, 54, of Newburgh; James G. Lovell, 58, of Cold Spring; James M. Ferrari, 59, of Montrose; and Kisook Ahn, 35, of Queens.
Lovell, an audio technician who had worked the "Today" show and other NBC programs, was traveling to Manhattan to work on the Rockefeller Center Christmas tree, longtime friend Janet Barton said. The tree-lighting ceremony is Wednesday night.
"He always had a smile on his face and was quick to share a friendly greeting," ''Today" executive producer Don Nash said in a message to staffers.
The NTSB has been urging railroads for decades to install Positive Train Control technology. In 2008, Congress required dozens of railroads, including Metro-North, to do so by 2015.
The MTA awarded $428 million in contracts in September to develop the system for Metro-North and its sister Long Island Rail Road.
But the MTA has asked for an extension to 2018, saying it faces technological and other hurdles in installing such a system across more than 1,000 rail cars and 1,200 miles of track.
"This incident, if anything, heightens the importance of additional safety measures like that one," said Sen. Richard Blumenthal of Connecticut, which is also served by Metro-North. "I'd be very loath to be more flexible or grant more time."
MTA spokeswoman Margie Anders said the agency began planning for a PTC system as soon as the law was put into effect.
"It's not a simple, off-the-shelf solution," she said.
The derailment came amid a troubled year for Metro-North, and marked the first time in the railroad's 31-year history that a passenger was killed in an accident.
In May, a train derailed in Bridgeport, Conn., and was struck by a train coming in the opposite direction, injuring 73 passengers, two engineers and a conductor. In July, a freight train full of garbage derailed near the site of Sunday's wreck.
Eltman reported from Mineola, N.Y. Associated Press writers Kiley Armstrong, Verena Dobnik, Deepti Hajela, Ula Ilnytzky, Colleen Long, Jake Pearson and Jennifer Peltz contributed to this report.