A New York-bound train that derailed, killing at least seven people and injuring more than 200, may have been travelling at more than twice the speed limit when it crashed, according to reports.
Meanwhile, the engineer at the controls of the train refused to talk to police yesterday.
The train from Washington may have been speeding at more than 100mph as it entered the sharp curve where it derailed on Tuesday night, according to two people described by the 'Wall Street Journal' as having knowledge of the investigation.
The speed limit on that section of track in Philadelphia drops to 50mph. Several other media outlets reported that investigators were focusing on the speed of the train as they analysed the "black box" data recorder recovered from the wreckage.
Survivors of the devastating crash on America's busiest rail route said they were stunned that the death tally was not higher, as they described the horrific scenes when fellow passengers were catapulted into luggage racks.
The fatalities included Jim Gaines (48), an Associated Press video software designer, who has two children; and Justin Zemseer, a young US Navy Academy midshipman. Fears were also growing for Rachel Jacobs (39), the head of a technology company, who was not heard from after texting her husband to say that she was about to board the train.
Four of the victims were found inside the wreckage, two bodies were recovered outside after being thrown from the train and another victim died in hospital.
The train tore apart as it flew off the tracks, with the engine separating from the seven carriages as passengers, bags and computers were sent hurtling through the air.
One carriage was so badly mangled that it resembled a crushed drink can.
Dozens of survivors scrambled to safety through the overturned carriages.
"I cannot believe how many of us walked away from this," said Jeremy Wladis, a businessman.
The tragedy is the third major crash in 12 months involving Amtrak, the federally-subsidised US intercity rail service.
While human error is being investigated as a possible cause, the crash also focused attention on the poor condition of rail infrastructure in the US.
Philadelphia's mayor said the train's data recorder has been recovered, but officials warned that the number of people unaccounted for was not yet confirmed and that search-and-rescue work was taking precedence over the investigation.
"It's a devastating scene," said Robert Sumwalt of the National Transportation Safety Board.
The rail corridor between New York and Washington remained closed as investigators looked for what went wrong. US President Barack Obama called the derailment "a tragedy that touches us all."
The engineer, who has not been identified, declined to provide a statement to authorities and left a meeting with an attorney, Philadelphia police said.
Passenger Jillian Jorgensen (27) said the train was going "fast enough for me to be worried" when it began a hard bank to the right. The train derailed, and the lights went out. Jorgensen said she "flew across the train" and landed underneath some seats.
Jorgensen said she managed to wriggle free as passengers screamed. She saw one man lying still, his face covered in blood, and a woman with a broken leg.
"It was terrifying and awful, and as it was happening it just did not feel like the kind of thing you could walk away from, so I feel very lucky," Jorgensen said.