Crawling through the wreckage to a field, the badly wounded Fonseca immediately began warning injured and stunned riders to avoid stepping on downed electrified wires.
Train engineer Brandon Bostian, 32, who suffered a concussion, told investigators he has no memory of what occurred after the train pulled out of the North Philadelphia station, just before the crash.
The National Transportation Safety Board is investigating.
Meanwhile, on Tuesday, railroad unions are urging Amtrak to put a second engineer in locomotives in the wake of the deadly derailment.
The Brotherhood of Locomotive Engineers and Trainmen and SMART Transportation say two engineers would serve as a check and balance on each other. Bostian was alone in the locomotive of Train 188 when it derailed.
Investigators are looking into why the train was going more than double the 50 mph limit around a sharp curve.
Federal authorities said they are uncertain if anything struck the windshield of the Amtrak train before it derailed, but they have not ruled out the possibility.
Investigators, however, said they are certain a gunshot did not strike the train.
FBI agents performed forensic work on a grapefruit-sized fracture on the left side of the Amtrak locomotive's windshield, and the National Transportation Safety Board said they found no evidence of any damage that could have been caused by a firearm.
Amtrak resumed service Monday with a 5:30 a.m. southbound train leaving New York City.
All Acela Express, Northeast Regional and other services also resumed service. Amtrak officials said Sunday that trains along the Northeast Corridor from Washington, D.C., to Boston would return to service in "complete compliance" with federal safety orders.
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