US experts have begun to probe how and why a New York City commuter train derailed over the holiday weekend, killing four, injuring more than 60, and nearly plunging into a freezing river.
The train may have been speeding when it veered off the rails in the Bronx borough at around 7.20am (2320 AEDT) on Sunday as it headed south to Grand Central Station in Manhattan.
The New York Fire Department said four people were killed, 11 others seriously injured, and another 56 suffered minor wounds.
Some passengers were "impaled" by debris as train cars flew into the air, officials said, while others had to be cut free from tangled metal.
"People were screaming," Joel Zaritsky told The New York Times. "I found myself thrown to the other side of the train."
Many survivors had broken limbs or injuries to their heads or necks. Some were led away with bloodied faces.
Investigators combed the scene and announced that a "multi-disciplinary team" would probe everything from the condition of the tracks to the signalling systems and the brakes.
"Our mission is to understand not just what happened, but why it happened, with the intent of preventing it from happening again," Earl Weener, a National Transportation Safety Board official, told reporters.
Weener said the train's event recorder - similar to an airplane's "black box" - had been recovered.
The data "will say how fast the train was travelling and whether or not the brakes were applied," New York Governor Andrew Cuomo said in earlier remarks to broadcaster MSNBC.
Speaking alongside Weener, Cuomo said a crane was on its way to the scene to right the derailed train cars.
That, according to Weener, was to check for any more victims and stop fuel from leaking from the locomotive.
Investigators will interview the train crew and conductor in the next days, Weener said. He expected investigators to remain on site for up to 10 days.
The Times, citing a senior city official who spoke on the condition of anonymity, reported that the train operator told first responders the brakes had failed. The account however had not been confirmed.
At least one passenger said the train was speeding as it took a curve on a downward slope leading into Spuyten Duyvil station, just north of Manhattan.
Before reaching the station, the train's seven cars derailed and flew across a grassy bank separating the railroad from the Hudson and Harlem rivers, which meet at that point.
The front car came to rest only a few feet from the water, and two cars toppled on their side.
Three of the dead were thrown from the train, police said, and divers searched the rivers in case other passengers were hurled into the frigid water.
Officials said there were some 120 passengers aboard the train when it flew off the tracks.
The Metropolitan Transportation Authority identified the four victims, who ranged in age from 35 to 59. Two were men, two were women, and all were New York-area US citizens.
