In the aftermath of Amtrak 188’s deadly derailment one year ago, investigators, journalists, passengers, and the public all sought some explanation for how Brandon Bostian, an experienced, skilled, and meticulous engineer, crashed his train in Philadelphia. Had there been a mechanical failure? Was Bostian too tired, or impaired for some reason, to concentrate? Had he been speaking on a cellphone? Was the locomotive hit by rocks?

But the National Transportation Safety Board, in findings revealed Tuesday morning, has concluded the cause of the crash was far simpler: Investigators believe Bostian was distracted by radio traffic about rocks striking another train at the worst possible moment, leading him to run the train far too fast through a curve and derail. NTSB Chairman Christopher Hart used that conclusion to make an impassioned call for full implementation of Positive Train Control, a technological fail-safe that’s intended to prevent collisions and derailments.

“An engineer who is not fatigued, distracted, or impaired is not infallible on their best day,” Hart said. “Our investigation is to see whether the engineer was backstopped by safety technology such as PTC, or positive train control. At the time of the accident, PTC was not implemented on the portion of track where the derailment occurred. If a PTC system has been active, this treatment not have derailed. Close to 200 passengers would not have been injured and 8 other passengers would still be alive today.”

The NTSB released a huge tranche of documents in the public docket for the case in February, but offered little in the way of conclusions about the cause of the accident.

The theory that Bostian might have been affected by rocks hitting the train emerged early on. There’s a logic to it: Trains are often struck by rocks, especially in the urbanized corridors of the Northeast. Besides, another train—a regional Philadelphia SEPTA train—had been “rocked” the same evening, and Amtrak 188 passed that train where it had pulled off, in part because the engineer had been injured by glass after being struck. NTSB concluded that though the windshield of Amtrak 188’s locomotive was cracked, the train had not been rocked.

Instead, Bostian was distracted by the radio traffic concerning the SEPTA train, which apparently prevented him from throttling down at a crucial moment. The area where Amtrak 188 derailed features several stretches where a train speeds up and slows down, but instead of slowing down when it should have, the train sped up to 106 miles per hour just before derailing. Investigators speculated that though Bostian knew the route well, his distraction might have led him to believe he had already passed the curve, so that he sped up too soon.

Bostian remains an enigma—the tragic figure at the center of the disaster. He cooperated with investigators, and reported having no memory of the time leading up to the crash, though the NTSB reported that the engineer had been moving the throttle during that stretch. He was concussed in the accident. Obsessed with trains since his childhood, Bostian had worked to achieve his dream job as engineer. Colleagues interviewed after the crash recalled him as an excellent engineer, toxicology tests came back negative, and his phone was turned off at the time of the derailment. The train was on time, and there was no indication that Bostian was trying to make up for lateness, a factor in some legendary train wrecks.

While the NTSB ruled out mechanical failure as a cause for the crash, it also found that windows on the train popped out, and that if the windows had remained in place, some passengers would not have been ejected, and might not have been as badly injured.

The fact that an engineer like Bostian could be at the controls for a horrific accident is chilling, but as Hart noted, it’s not the only case. There were deadly rail crashes in California in September 2008 and the Bronx in December 2013, both of which would likely have been avoided by PTC. In total, NTSB has calculated that PTC would have prevented 145 accidents since 1969, in which 288 people were killed and nearly 6,600 were injured.

Amtrak has since implemented PTC on the stretch in Philadelphia where the crash occurred, as well as throughout most of the Northeast Corridor. Generally, however, the United States has been slow to adopt the technology. In 2008, Congress mandated that every railroad with intercity passenger service, commuter service, or hazardous materials install PTC by the end of 2015. But railroads have pushed back on the mandate. Amtrak, which is perpetually strapped for cash, said it could not afford to meet the deadline. Commercial railroads have lobbied aggressively against the deadline as well, and late last year Congress agreed to extend the timetable by at least three years, to the end of 2018.

“As we discussed our findings regarding this preventable tragedy, let us keep in mind the deadline that matters is not 2018. It is not some later date made possible by a conditional extension,” Hart warned on Tuesday. “The deadline that really matters is the date of the next PTC-preventable tragedy.”

As the board voted on conclusions, in fact, Vice Chair Bella Dinh-Zarr, citing the importance of PTC, suggested relegating engineer error from the probable cause of the accident to contributing cause, while naming the lack of PTC as the probable cause. NTSB staff, however, opposed the switch, as did Hart, noting that the engineer is still responsible for following rules and that PTC is a safety net; Dinh-Zarr’s attempt was defeated.

That minor disagreement aside, the takeaway from the NTSB hearing was the importance of the new technology. But as Dinh-Zarr pointed out, NTSB has been advocating for PTC since 1970. So far, neither railroads nor regulators are listening closely.