The Philadelphia Fire Department said it had sounded its fourth alarm, indicating a large-scale operation for passengers of Amtrak Train 188 traveling from Washington and bound for New York yesterday.
At least one of the metal train cars was bent in an L-shape.
Dozens of emergency personnel used flashlights to comb the area for survivors after dark. They used ladders to climb over the flipped trains.
Multiple fire engines, ambulances and police cars were deployed on the scene.
The fire department said it had requested helicopters for additional lighting.
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There are no seatbelts on trains operated by Amtrak, the national long-distance rail service.
The National Transportation Safety Board said it was gathering information about the derailment.
Former US Congressman Patrick Murphy of Pennsylvania was on the train.
"I am ok. Helping others. Pray for those injured," he tweeted.
Murphy, who was on board the cafe car, said the train seemed to be going 97-113 kilometers per hour when it suddenly derailed and rolled.