National Transportation Safety Board member Robert Sumwalt said an assistant conductor aboard the train told investigators that she heard the Amtrak engineer talking over the radio with an engineer for a regional railroad just before the crash.
The regional engineer, who was in the same area as the Amtrak train, said his train had been hit by a rock or some other projectile. The conductor heard Brandon Bostian, who was at the Amtrak controls, say the same had happened to his train, according to Sumwalt.
The windshield of the Amtrak train was shattered in the accident but one area of glass had a breakage pattern that could be consistent with being hit by an object and the FBI is investigating, he said.
Sumwalt declined to speculate about the exact significance of a projectile, but the idea raised the possibility that the engineer might have been distracted, panicked or even wounded in the moments before the train left the rails.
The Southeastern Pennsylvania Transportation Authority does not yet know what caused the damage to its train that night, said Jerri Williams, a spokeswoman for the agency.
SEPTA trains travelling through the area — including one of the poorest and most violent parts of Philadelphia — have had projectiles thrown at them in the past, whether by vandals or teenagers, she said. It was unusual that the SEPTA train was forced to stop on Tuesday night.
NTSB spokesman Peter Knudson said the board was seeking more information about a third report of damage that night, this one involving a different Amtrak train.
Not long before the derailment, two passengers on a southbound Amtrak told The Philadelphia Inquirer that something shattered a window on their train as it passed through the same area. They said Amtrak police boarded the train at 30th Street station in Philadelphia to document the incident.
The derailment has made it clear that despite the train industry's widespread use of electronic signals, sensors and warning systems, safety still sometimes comes down to the knowledge and experience of the engineer.
Those skills would have been critical on the curve where the New York-bound train derailed, killing eight and injuring more than 200 in the deadliest U.S. train accident in nearly six years.
Instead of high-tech signals or automatic controls, engineers on that stretch of track have to rely on their familiarity with the route and a printed timetable they carry with them, not unlike engineers a century ago.
"We're depending heavily on the human engineer to correctly obey and interpret the signals that he sees and also speed limits and other operating requirements," said David B. Clarke, a railroad expert at the University of Tennessee.
The engineer, who had been working that route for several weeks, told investigators that he does not recall anything after ringing the train's bell as he passed by the North Philadelphia station a couple of minutes before Tuesday night's crash.
The engineer said in an NTSB interview Friday that he felt comfortable with the train and was not fatigued, Sumwalt said.
In the minute before the derailment, the Amtrak train accelerated from 70 mph to more than 100 mph, even though the curve where it came off the tracks has a maximum speed of 50 mph.
It's not clear whether Bostian manually accelerated, Sumwalt said, though a data recorder shows that he did engage a braking system seconds before the derailment.
Experts say the railroad's signalling system would have slowed the train automatically if it had hit the maximum speed allowed on the line, but older cab-signal and train-control systems do not respond to localized speed restrictions.
Investigators are also conducting drug tests. Bostian's lawyer has said he was not using drugs or alcohol.
James Weir, a friend of Bostian's since they were teenagers in the Memphis area, said he called the engineer after hearing about the wreck, but that his friend was hospitalized and could not say much.
As a teenager, Bostian was a safe driver who would not go even 5 mph over the speed limit, he said.
"Whenever I would drive, I'd tend to go a little over and he'd fuss at me. He'd tell me to slow down. He's just not the kind of guy that breaks the rules," Weir said. "He puts safety ahead of everything."
Preliminary checks have not found any pre-existing problems with the train, the rail line or the signals.
Because of his experience, Bostian should have known the route, even if there's not so much as a speed limit sign on the side of the tracks, said Howard Spier, a Miami-based lawyer who is a former president of the Academy of Rail Labor Attorneys.
"It's engrained in them. He knew it," Spier said. "I'm convinced he knew he was entering a speed-restrictive curve."
The wreck has focused new attention on positive train control, a system that automatically brakes trains going too fast. It is installed on the tracks where the train derailed, but it had not been turned on because further testing was needed, Amtrak President Joseph Boardman said.
Boardman said this week that he intends to have the system running across Amtrak by the end of this year, as Congress mandated back in 2008.
The system is already operating in other parts of the Northeast Corridor, the busy stretch of tracks between Boston and Washington. An older, less robust automatic-control system is in place for southbound trains in the same area as the derailment.
Associated Press writers Geoff Mulvihill in Haddonfield, New Jersey, and Josh Cornfield in Philadelphia and Matt Friedman in Washington contributed to this report.Suggest a correction