The engineer of an Amtrak train that derailed in Philadelphia last year, killing eight people, went "from distraction to disaster" in a matter of seconds because he was so focused on radio traffic about a commuter train that had been hit by a rock, federal investigators said Tuesday.

Brandon Bostian lost track of where he was and accelerated to 106 mph as his train approached a sharp curve with a 50 mph speed limit, investigators said at a National Transportation Safety Board hearing convened to pinpoint the cause of the tragedy.

Bostian told investigators after the wreck that he remembered radio traffic from a commuter train operator who said a rock had shattered his windshield.

Investigators said the crash would not have happened had the stretch of track been equipped with Positive Train Control, which automatically slows a train that is exceeding the speed limit. Such equipment is now in place there and along much of the rest of the Northeast Corridor.

If PTC had been in use, "we would not be here today," said Ted Turpin, an NTSB investigator.

The chain of events also illustrated the potential for tragedy when people throw rocks at trains - a problem railroads are almost powerless to stop but is so common the industry has a term for it: "getting rocked."

Investigators suggested Tuesday that Bostian was so consumed by radio chatter about the rock-throwing that he didn't realize he was approaching the sharp curve.

Bostian was monitoring radio communications between the Southeastern Pennsylvania Transportation Authority train's engineer and dispatch until about a minute before his Amtrak train reached 106 mph, said Steve Jenner, an NTSB investigator.

Bostian's full-throttle acceleration would have made sense for someone who thought he had already passed the curve, Jenner said. After the curve, the tracks open up into a straightaway with a speed limit of 110 mph.

NTSB board member Robert Sumwalt said: "He went, in a matter of seconds, from distraction to disaster."

Bostian, who has been suspended without pay since the crash for speeding, did not attend the hearing. His attorney didn't return an email sent Monday seeking comment. An Amtrak spokesman said the agency would comment after the hearing.

Bostian had told investigators that he didn'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 suffered during the wreck probably prevented him remembering what happened, NTSB medical officer Mary Pat McKay. She said he was thrown around the cab of his locomotive when it overturned.

Early in the investigation, the NTSB focused on whether the Amtrak train had 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 in either NTSB interview that his train had been struck, too.

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 Positive Train Control 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 a deadline at the end of 2018.

After the derailment, the train's emergency windows dislodged as the train cars slid on their sides, killing four people who were ejected, according to NTSB investigator Dana Sanzo.

Sign up for the NJ1015.com Newsletter

Get the best of NJ1015.com delivered to your inbox every day.

(© 2016 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed)