Joel Zaritsky was dozing as he traveled to a dental convention aboard the train. He woke up to feel his car overturning several times.
"Then I saw the gravel coming at me, and I heard people screaming," he told The Associated Press, holding his bloody right hand. "There was smoke everywhere and debris. People were thrown to the other side of the train."
In their efforts to find passengers, rescuers shattered windows, searched nearby woods and waters and used pneumatic jacks and air bags to peer under wreckage. Crews planned to bring in cranes during the night to right the overturned cars on the slight chance anyone might still be underneath, National Transportation Safety Board member Earl Weener said.
The agency was just beginning its search into what caused the derailment, and Weener said investigators had not yet spoken to the train conductor, who was among the injured.
Meanwhile, thousands of people braced for a complicated Monday morning commute, with shuttle buses ferrying passengers to another line.
Investigators were due to examine factors ranging from the track condition to the crew's performance. Gov. Andrew Cuomo said the track did not appear to be faulty, leaving speed as a possible culprit for the crash. The speed limit on the curve is 30 mph, compared with 70 mph in the area approaching it, Weener said.
Authorities did not yet know how fast the train was traveling but had found a data recorder, he said.
One passenger, Frank Tatulli, told WABC-TV that the train appeared to be going "a lot faster" than usual as it approached the sharp curve near the Spuyten Duyvil station.
Nearby residents awoke to a building-shaking boom. Angel Gonzalez was in bed in his high-rise apartment overlooking the rail curve when he heard the roar.
"I thought it was a plane that crashed," he said.
Mike Gallo heard the same noise as he was walking his dog. He looked down at the tracks, saw injured people climbing out of the train and "knew it was a tragedy right away."
Within minutes, dozens of emergency crews arrived and carried passengers away on stretchers, some wearing neck braces. Others, bloodied and scratched, held ice packs to their heads.
Two men and two women were killed, said the Metropolitan Transportation Authority, which runs the railroad. Three of the dead were found outside the train, and one was found inside, authorities said. The victims' names had not yet been released. Autopsies were scheduled for Monday, said the New York City medical examiner's office.
Eleven of the injured were believed to be critically wounded and another six seriously hurt, according to the Fire Department.
To Cuomo, the scene "looked like a toy train set that was mangled by some super-powerful force," the governor said in a phone interview with CNN.
As deadly as the derailment was, the toll could have been far greater had it happened on a weekday, or had the lead car plunged into the water instead of nearing it. The train was about half-full at the time of the crash, rail officials said.
"On a workday, fully occupied, it would have been a tremendous disaster," New York City Fire Commissioner Salvatore Joseph Cassano told reporters at the scene. The affected line, called the Hudson line, carries about 18,000 people on an average weekday morning.
For decades, the NTSB has been urging railroads to install technology that can stop derailing caused by excessive speed, along with other problems.
A rail-safety law passed by Congress in 2008 gave commuter and freight railroads until the end of 2015 to install the systems, known as positive train control. Aimed at preventing human error — the cause of about 40 percent of train accidents — it can also prevent trains from colliding, entering tracks undergoing maintenance or going the wrong way because of a switching mistake.
But the systems are expensive and complicated. Railroads are trying to push back the installation deadline another five to seven years.
Metro-North is in the process of installing the technology. It now has what's called an "automatic train control" signal system, which automatically applies the brakes if an engineer fails to respond to an alert that indicates the speed is excessive.
Such systems can slow trains in some circumstances but not bring them to a halt, said Grady Cothen, a former Federal Railroad Administration safety official.
Sunday's accident came six months after an eastbound train derailed in Bridgeport, Conn., and was struck by a westbound train. The crash injured 73 passengers, two engineers and a conductor. In July, a freight train full of garbage derailed on the same Metro-North line near the site of Sunday's wreckage.
"Safety is clearly a problem on this stretch of track," state Sen. Jeff Klein, who represents the nearby area, said Sunday.
Earlier this month, Metro-North's chief engineer, Robert Puciloski, told members of the NTSB investigating the Bridgeport derailment and that the railroad is "behind in several areas," including a five-year schedule of cyclical maintenance that had not been conducted in the area of the Bridgeport derailment since 2005.
The NTSB issued an urgent recommendation that Metro-North use "redundant protection," such as a procedure known as "shunting," in which crews attach a device to the rail in a work zone alerting the dispatcher to inform approaching trains to stop.