Emergency personnel said 65 people were hurt, six of them critically, after the accident on the train heading from Washington, D.C. to New York. Others walked away from the crash with light injuries.
The train’s seven train cars, including the engine car, were crushed, turned over on their side or upside down in the late evening disaster.
One was unrecognizable as a train car, as it lay on the ground in a ruined mass of metal.
As night fell, rescuers with flashlights gingerly scoured through the remains.
“It is an absolute disastrous mess,” Philadelphia Mayor Michael Nutter told reporters. “I have never seen anything like this in my life.”
Officials declined to speculate on the cause of the incident, though some experts suggested the crash may have been caused due to a track defect or wheel failure.
Witnesses said the front of Amtrak Train 188 shook as it went into a turn, and the six cars behind it then went off the rails.
An estimated 243 people, including five crew members, were aboard the train when it crashed around 9:30 pm (0130 GMT Wednesday).
Nutter warned that the casualty and injury estimates were only preliminary, hinting at the potential for a higher toll.
He also would not confirm whether all those aboard the train had been accounted for.
Hydraulic tools had to be used to remove passengers from some of the most badly damaged train cars, firefighters said.
“I’ve never seen anything so devastating. They are in pretty bad shape,” said Fire Commissioner Derrick Sawyer, referring to the train cars.
“You can see they completely, completely derailed from the track, destroyed completely and they’ve been overturned completely.”
Passengers recalled the chaos of the derailment.
Former US Congressman Patrick Murphy of Pennsylvania, who was on the train, said he was sitting on a bench in the cafe car when the train began to topple.
“It went to my right, then to my left. Everyone who was on the left side of the car, where I was sitting, just got thrown completely over to the right side.”
Murphy tweeted a photo from inside a wrecked train car. He said he was uninjured and trying to assist other passengers.
Murphy said the train seemed to be going 60-70 miles per hour (around 100-110 kilometers per hour) when it suddenly derailed and rolled. Passengers had to kick out a window to escape.
Some of the injured — many with bloodied hands and faces — were unable to move, Murphy said.
Another passenger, Jeremy Wladis, 51, was on the last car of the train when he felt the jolt. He said he saw “phones, laptops, everything flying,” the Philadelphia Inquirer newspaper reported.
“There were women launched up in the luggage rack,” he said. “I don’t even know how they got there.”
Hundreds of emergency personnel, including firefighters and police, were deployed.
Emergency crews struggled to search for survivors in the darkness, at first relying on flashlights to comb the area. They were later aided by helicopters hovering overhead.
The workers used ladders to climb over the flipped trains.
All service by Amtrak, the national long-distance rail system, was canceled for the rest of the night between Philadelphia and New York.
Nutter said the rails were “completely wiped out” in the area of the accident, and that he did not anticipate service through Philadelphia for the rest of the week.
At least one of the train cars appeared crushed and turned on its side. A large metal beam was rammed into another car, though it was unclear whether it had fallen onto the car or the car had crashed into it.
The engine car completely separated from the rest of the train and one of the cars was perpendicular to the rest.
The train had so much force at the time of the crash that it bent the sturdy rail tracks in at least one area.
Train 188, a Northeast Regional rail service train, was scheduled to leave Washington at 7:10 pm (2310 GMT) and arrive in New York at 10:34 pm.
There are no seatbelts on trains operated by Amtrak.
The US Department of Homeland Security, the National Transportation Safety Board, the Southeastern Pennsylvania Transportation Authority and other agencies were involved in trying to understand the causes of the crash, along with Amtrak.
NTSB personnel were headed to the scene of the crash and expected to arrive Wednesday.