Crashed NY Metro-North train was 'over speed limit'
A train that crashed on Sunday in New York City was going 82mph (132 km/h) in a 30mph zone when it ran off the rails, investigators have said.
Four people were killed and more than 60 injured in New York's Bronx borough.
The Metro-North train from Poughkeepsie to New York City crashed about 07:20 local time (12:20 GMT) on a curve.
The National Transportation Safety Board (NTSB) is investigating whether a mechanical failure or human error caused the train's high speed.
'So many questions'
The train appeared to have been travelling over the speed limit even before it thundered into the curve where it crashed - the speed limit on the track approaching the curve is 70mph, NTSB board member Earl Weener said on Monday.
New York Senator Charles Schumer called the locomotive's speed "frightening".
"The fact that it was going 82mph even before the curve raises so many questions, and it's scary," Mr Schumer said, adding that the rail line's tracks and signals appeared to have been in working order at the time of the accident.
Mr Weener said the train had made nine stops prior to the derailment, and officials had not yet uncovered any brake problems.
He said preliminary data from the train's event recorders indicated the train's brakes became fully engaged just five seconds before the locomotive came to a complete stop after the crash.
In addition, he said the locomotive's throttle went to idle six seconds before the train came to a stop.
Authorities continue to question the train's engineer and three other crew members, and planned to transport the locomotive to a secure facility for further examination.
Officials were examining the engineer's mobile phone - a standard procedure, Mr Weener said.
The engineer and assistant conductor were both injured in the crash.
Authorities also obtained surveillance video of the derailment from a nearby bridge, but the picture was of low quality, he added.
Officials have identified the deceased as Donna Smith, 54, James Lovell, 58, James Ferrari, 59, and Ahn Kisook, 35. Three of the dead were found outside the train, and one was found inside.
Of the 150 or so who were on board, 11 remain critically injured.
One man suffered a spinal cord injury that could leave him paralysed from the neck down, a doctor told CNN.
Connecticut Senator Richard Blumenthal said the accident had raised questions about the safety and reliability of the Metro-North commuter rail line.
"Riders are losing patience, and so are a lot of us as public officials," he said.
Some 26,000 weekday commuters on the route were warned to brace for crowded trains on the first morning after the derailment.
Train service south of Spuyten Duyvil, the Bronx neighbourhood where the crash occurred, remains suspended, Metro-North said on Monday morning. Buses were being provided to the New York subway system.
The accident was the second passenger train derailment this year for Metro-North, which, until Sunday, had never experienced a passenger death in an accident in its 31-year history.
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.
A freight train derailed near the same location as Sunday's accident in July, damaging about 1,500ft (457m) of track. No-one was hurt.