The National Transportation Safety Board also put some of the blame on the railroad industry's decades-long delay in installing Positive Train Control, equipment that can automatically slow trains that are going over the speed limit.
Engineer Brandon Bostian was apparently so focused on the rock-throwing he heard about over the radio that he lost track of where he was and accelerated full-throttle to 106 mph as he went into a sharp curve with a 50 mph limit, investigators said at an NTSB hearing convened to pinpoint the cause of the May 12, 2015, tragedy. About 200 people aboard the Washington-to-New York train were injured.
"He went, in a matter of seconds, from distraction to disaster," NTSB member Robert Sumwalt said.
Bostian, who has been suspended without pay since the crash for speeding, did not attend the hearing. He and his lawyer did not immediately return calls and emails seeking comment.
Had Positive Train Control been in use along the stretch of track, "we would not be here today," said Ted Turpin, an NTSB investigator.
"Unless PTC is implemented soon," NTSB chairman Christopher Hart warned, "I'm very concerned that we're going to be back in this room again, hearing investigators detail how technology that we have recommended for more than 45 years could have prevented yet another fatal rail accident."
Among other things, the NTSB also recommended research into seat belts in railcars and ways to secure luggage that can become missiles in a derailment; training for railroad crew members on multitasking; and the use of new equipment and procedures to help engineers keep track of their location in spots where there is no Positive Train Control.
In a statement, Amtrak said it has "taken full responsibility for and deeply regrets the tragic derailment" and will carefully review the NTSB findings and recommendations and quickly adopt them where appropriate.
Amtrak noted that Positive Train Control is already in place on most of its portion of the Northeast Corridor and that it has also installed inward-facing video cameras on locomotives.
The problem of people throwing rocks at trains is so common that train crews have a term for it: "getting rocked." But it is a danger railroads are almost powerless to stop. No one was ever arrested in the rock-throwing in Philadelphia.
Bostian told investigators after the wreck that he remembered radio traffic from a Philadelphia commuter train operator who said a rock had shattered his windshield. He was monitoring the radio traffic until about a minute before his Amtrak train reached peak speed, and at one point passed the commuter train on an adjoining track, investigators said.
Investigators said they believe Bostian was accelerating because he thought he had already passed the sharp Frankford Junction curve. After the curve, the tracks open up into a straightaway where the speed limit is 110 mph.
During the investigation, authorities ruled out cellphone use on Bostian's part, as well as drugs or alcohol.
"Excluding all the other suspects that we looked at, the best we could come up with was that he was distracted from this radio conversation about the damaged train and forgot where he was," Hart said.
Bostian told investigators he couldn't recall what happened between pushing the throttle to pick up speed and then braking when he felt the train going too fast into the curve. A blow to the head when he was thrown around the cab of his overturned locomotive probably affected his memory, NTSB medical officer Mary Pat McKay.
Early in the investigation, the NTSB focused on whether the Amtrak train had also been hit with a rock or other projectile minutes before the crash. But investigators confirmed Tuesday that it was not.
Bostian told investigators that he was concerned about the welfare of the commuter train's engineer and "a little bit concerned" for his own safety, but he never indicated that his train had been struck, too.
Duy Nguyen, of Teaneck, New Jersey, a passenger who suffered a cut on his head and fractures in his back, attended the NTSB hearing. The Temple University professor said he was stunned by the findings.
"The part that doesn't make sense is how does one accelerate when you're distracted?" Nguyen said. "The inclination is to slow down." He added: "Part of me is mad at Amtrak. Part of me is resigned that there's something that happened and you have to endure and survive and move on."
Bostian, known among his friends for his safety-mindedness and love of railroading, apparently commented in an online forum for train enthusiasts on a range of industry issues, including safety. Some of the posts lamented that railroads hadn't been fast enough to adopt Positive Train Control.
Amtrak has installed PTC on all the track it owns on the Northeast Corridor from Boston to Washington. A 56-mile stretch from New Rochelle, New York, to New Haven, Connecticut, is owned by other entities and is expected to have automatic controls installed by the end-of-2018 deadline.
The southbound stretch of track near the accident site had an earlier-generation type of automated control for slowing trains. But the northbound stretch, where the wreck happened, did not. The more-advanced PTC had been installed there but was still being tested at the time of the crash.
The investigation also pointed up the need to make passenger trains safer. In the derailment, the train's emergency windows dislodged as the cars slid on their sides, and four people were ejected and killed, according to investigators.