Four people have been killed and more than 60 injured in a passenger train crash in the Bronx area of New York.
Eleven of the injured are believed to be in a critical condition in hospital.
The Metro-North train's locomotive and carriages derailed as the train went into a bend in the railway line near Spuyten Duyvil station.
At least one eyewitness said the train - the 05:54 from Poughkeepsie to Grand Central Station - was travelling much faster than normal at the time.
None of the carriages went into the adjacent Hudson or Harlem rivers - although pictures from the scene suggest this nearly happened.
The train appeared to be going "a lot faster'' than normal as it approached the bend coming into the station, passenger Frank Tatulli told WABC-TV.
Metropolitan Transportation Authority spokeswoman Marjorie Anders said the bend where the derailment occurred was in a slow speed area, and that the train's "black box" recorder should be able to tell how fast the train was travelling.
Later on Sunday, federal investigators said the derailed carriages would be turned upright to check for any other possible victims.
National Transportation Safety Board official Earl Weener said his teams would be on site for several days documenting evidence.
"Our mission is not just to understand what happened but why it happened, with the intent of preventing it happening again," he said.
Joel Zaritsky had been travelling on the train to attend a dental convention in New York City.
"I was asleep and I woke up when the car started rolling several times," he told the Associated Press news agency.
"Then I saw the gravel coming at me and I heard people screaming. There was smoke everywhere and debris. People were thrown to the other side of the train.''
Felix and Mabel Lam, Cuban immigrants who have lived in the Bronx for 42 years, were woken by the sound of the crash.
Mabel spoke of a "rushing sound".
Felix rushed down to the tracks and saw the injured staggering in a daze from the wrecked carriages, many bleeding.
Speaking to the media near the scene, New York state Governor Andrew Cuomo confirmed the casualty figures, adding that the National Transportation Safety Board (NTSB) was beginning an investigation.
"It's obviously a very tragic situation," he said. "The first order of business is to care for the people who were on the train... we'll wait to see what the NTSB says before speculating as to any causes."
A section of line between the Bronx and part of Westchester County could be closed for a week or more and Governor Cuomo warned commuters to expect long delays.
"I think it's fair to say that tomorrow people who use this line should plan on a long commute or plan on using the Harlem line," he said.
Metro-North is a rail service that serves commuters from New York City's northern suburbs. It is not part of the New York City subway system.
Sunday's accident is the second passenger train derailment this year for the rail service.
On 17 May, an eastbound train derailed in Bridgeport, Connecticut, and was struck by a westbound train. The crash injured 73 passengers, two engineers and a conductor. Eleven days later, a track foreman was struck and killed by a train in West Haven, Connecticut.