WASHINGTON - The engineer who derailed an Amtrak train lost his bearings shortly before he accelerated into a dangerous Philadelphia curve last year, likely because he was distracted by radio talk about a SEPTA train struck by rocks, federal investigators concluded Tuesday.

Their report offered the clearest explanation yet for the May 12 crash of a New York City-bound train that killed eight people and injured about 200.

"The engineer's world is one of fallible human decisions and actions in an imperfect environment," said Christopher Hart, chairman of the National Transportation Safety Board. It is one "in which a loss of awareness can take a terrible toll."

The NTSB voted to accept the conclusion that Amtrak engineer Brandon Bostian lost his "situational awareness" and sped to 106 mph shortly before entering a curve on which the speed limit was 50 mph, causing the derailment.

The NTSB concluded that Bostian likely thought he was on a stretch of track beyond the Frankford Curve, where the speed limit was 110 mph.

He "went in a matter of seconds from distraction to disaster," said NTSB member Robert Sumwalt. He called it "a very basic error."

The nearly three-hour NTSB hearing covered the actions of the engineer, structural weaknesses in the train cars that may have contributed to the injuries and deaths, and communications issues among emergency responders that overburdened some Philadelphia hospitals with patients.

The board also said the incident, and others like it, showed an urgent need to install an upgraded rail safety system, Positive Train Control (PTC), throughout the country. Amtrak has done so along all of the Northeast Corridor tracks it controls, but most rail lines have lagged.

"The government and industry have not acted for decades on a well-known safety hazard," said Bella Dinh-Zarr, the NTSB's vice chair.

Bostian's lawyer, Robert Goggin, did not respond to a call for comment Tuesday.

The engineer has told the NTSB he could not remember the specifics of why he accelerated - investigators said head injuries can lead to amnesia - but the board surmised from its interviews with Bostian that concern about the SEPTA train and its shattered windshield was the most likely distraction.

For six minutes after Train 188 left 30th Street Station, Bostian closely monitored about two dozen radio messages about the incident, the location of the disabled train, and whether the engineer needed medical attention, NTSB investigator Steve Jenner said.

The NTSB said he lost situational awareness - a term used by engineers and pilots referring to where they are, what is happening around them, and what comes next.

An engineer who loses that focus "may forget to take action when it is time to do so," said Steve Jenner, an NTSB investigator. In talking to investigators, Bostian did not describe the incident as something that had fractured his attention.

But, Hart said, "oftentimes the one who is lacking situational awareness is the last to know it."

The NTSB found no prior concerns in Bostian's record or training. One investigator called him qualified and experienced.

Rail engineers take pride in memorizing their routes - if one was blindfolded and left at a particular location, he or she could quickly identify it and the appropriate speed limit, NTSB staffers said.

But that reliance on memory, said investigator Ted Turpin, means "once you get lost, then you're in trouble."

Thomas Kline and Robert Mongeluzzi, attorneys who are representing 32 injured passengers, criticized the NTSB's conclusions and contended its investigators had failed to ask Bostian hard questions during two interviews last year.

"The one thing that we know that is not supposition, that is not speculation, is that he was going 106 mph in a 50-mph curve," Mongeluzzi said. "He may have lacked situational awareness, but he didn't lack situational awareness after the accident, when the police wanted to question him and he lawyered up and refused to answer their questions."

Added Kline: "There is no doubt in our minds that the conduct of Brandon Bostian on that fateful night was reckless. The conclusion that there was situational unawareness and that he may have somehow lost his way is truly rank speculation."

NTSB investigators said that the structure of the train cars themselves contributed to the human toll. As the cars toppled and skidded on their sides, their windows did not hold. Four passengers died after being ejected through the windows.

The information gathered in the wreckage and from passengers described everything thrown in violent motion as the train derailed. The force hurtled people, luggage, and seats freely about the cars, which contributed to many of the 46 serious injuries, said Dana Sanzo, one of the investigators.

Train cars are built to protect passengers from being propelled forward, Sanzo said, but do not protect against lateral motion. Investigators recommended train designs take lateral motion into account and take steps to secure baggage. They also recommended a federal study to evaluate whether seat belts could make trains safer.

Amtrak CEO Joseph Boardman said he supported the NTSB recommendations and said the rail agency would implement them.

The board stressed how preventable the derailment would have been had Positive Train Control been in place. The system, sought for decades by the NTSB, can slow or stop speeding trains. Since 2008, 37 people have been killed in accidents that could have been avoided if PTC had been in place, Hart said.

While Amtrak has installed it along all but 56 miles of the Northeast Corridor, including the stretch where this crash occurred, progress has been scant elsewhere, and last year Congress gave railroads even more time - until 2018 - to install the system on major freight and passenger lines nationwide. Extensions are also possible under the new law.

Originally, railroads were mandated to have PTC in place by the end of last year. Hart called on them to act more quickly.

"The deadline that really matters is not 2018, and it is not some later date made possible by an additional extension," he said. "The deadline that really matters is the date of the next PTC-preventable tragedy."

@JasmLaughlin

Staff writer Chris Mondics contributed to this article.