The speed was more than twice the approved limit, and the brakes barely slowed the train before the cars tumbled off the track late Tuesday, leaving more than 200 people injured, some with broken ribs and collapsed lungs.
The National Transportation Safety Board cautioned that its first assessment of the data was preliminary, and that it would need more time to piece together what happened to Amtrak Train 188, headed from Washington to New York.
"Our mission is to find out not only what happened, but why it happened, so that we can prevent it from happening again," the NTSB's Robert Sumwalt told reporters, saying his team would be on the ground for a week.
The engineer's "full emergency brake application" only slowed the train speed from 106 miles (170 kilometers) per hour to 102 mph, Sumwalt said, noting: "It takes a long time and distance to decelerate a train."
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He said a speed control system used along Amtrak's well-traveled Northeast Corridor between Washington and New York was not yet in place in the area of the crash.
"We feel that had such a system been installed in this section of track, this accident would not have occurred," he said.
Officials warned some of the 243 passengers and crew listed on the train's manifest -- many of whom limped away from the wreck bloodied and dazed -- had not yet been accounted for, meaning the death toll could still rise.
Sam Phillips, the city's director of emergency management, said area hospitals had treated "over 200 patients last night and this morning."
Herbert Cushing, the chief medical officer at Temple University Hospital, said some suffered rib fractures or collapsed lungs. Some of the victims hailed from Spain, Belgium, Germany and India.