Why did he accelerate into a curve?

Brandon Bostian, the engineer of the Amtrak train that crashed in Frankford in 2015, killing eight people and injuring more than 200, has been on trial for four days. More than a dozen witnesses have testified about the disaster — from Bostian’s training, to the injuries passengers suffered, to how police initially believed the derailment could have been a terrorist attack.

But no one has yet been able to explain why Bostian — who wasn’t under the influence of drugs or alcohol and wasn’t using his cellphone — sped up to 106 mph, twice the speed limit, heading into a dangerous curve.

Could he have been distracted? Disoriented? Or, as one of his attorneys asked a witness, did he simply lose track of where he was?

“He could have,” said Keith Strobel, an engineer who helped train Bostian to navigate the Northeast Corridor. “I wasn’t in the seat, though.”

The question — unanswered since the day of the crash nearly seven years ago — looms as prosecutors try to convince a jury that Bostian committed a crime. He is charged with causing a catastrophe, eight counts of involuntary manslaughter, and nearly 250 counts of reckless endangerment.

Prosecutors, led by Christopher Phillips of the state Attorney General’s Office, do not have to prove intent to harm to secure a conviction, which could send Bostian, 38, to prison for years.

Phillips, who rested his case early Wednesday afternoon, said Bostian’s acceleration into the curve on May 12, 2015, was criminally negligent — a reckless action that led to death and serious injuries. To bolster that argument, Phillips called engineers including Strobel to testify about how rigorous the training process is. Engineers have to memorize their route, Strobel said, showing the ability to identify where they are, and what the speed limit is, simply by looking at the area’s physical characteristics.

Bostian’s lawyers have sought to convince jurors that the crash was a tragic mistake made by an otherwise conscientious engineer. Strobel, even as he testified for the prosecution, said he was stunned that Bostian was involved in a crash, calling him an “excellent” engineer.

Brian McMonagle, one of Bostian’s attorneys, has said the derailment happened only after Bostian learned through radio chatter that another nearby SEPTA train made an emergency stop when it was struck by a rock, shattering the windshield and sending glass into the locomotive.

That train’s engineer, Curtis Parrish, testified this week, as did a passenger on another train, an Amtrak Acela, who said a rock or a battery struck his window around the same time and same area in the moments before Bostian’s train went careening off the rails.

On Wednesday, the Acela’s engineer, Herbert Harris, testified that a staffer on board had told him the train had been shot, which Harris subsequently reported over the radio. Still, Harris testified that he did not stop the train when it was struck, and he was allowed to continue on his route after a brief inspection at 30th Street Station.

Harris and several other engineers told jurors that trains being struck by projectiles was fairly common.

The defense and the prosecution drew differing conclusion from all this testimony. McMonagle said the “attacks” on other trains caused Bostian to be disoriented. Prosecutors said the incidents demonstrated that the other engineers were able to operate their trains safely despite the projectiles.

It was not clear if Bostian might testify. He has said he does not remember much about the moments leading up to the wreck. He told investigators with the National Transportation Safety Board that what little he does recall might actually be memories from a different day on the same route. A federal agent who testified this week read some of Bostian’s statements to the NTSB to the jury.

The NTSB, which investigated the crash at length, concluded that Bostian likely lost his “situational awareness” after hearing the radio chatter about rock-throwing. And McMonagle questioned several witnesses — including Jonathan Hines, an Amtrak compliance director — about that issue, asking if even careful engineers could lose their bearings or make mistakes while on the rails.

“Yes,” said Hines.

Bostian’s attorneys called several witnesses Wednesday, including a medical expert who said it was normal for people to experience some memory loss after a traumatic event, especially if the person also suffers a head injury, as Bostian did in the crash.

Closing arguments are expected Thursday. Jurors will then be tasked with determining Bostian’s fate.