Similarities were immediately drawn between the crashes, but after months of investigating, the NTSB now believes sleep apnea was involved in both.
Lawyers for the NJ Transit engineer have already cited his undiagnosed sleep apnea, a disorder previously linked to the 2013 derailment of a Metro North commuter train in the Bronx.
The LIRR derailment in January was similar to the NJ Transit crash, albeit less severe.
The Federal Railroad Administration recently decided that they would not require sleep apnea testing for truck drivers and train engineers -- although the MTA and other local commuter rail lines quickly said they would continue their existing testing programs anyway.
The engineers of the two commuter trains were both suffering from severe sleep apnea and have no memory of the crashes, according to investigative reports and interview transcripts made public Thursday.
Neither engineer had been diagnosed with sleep apnea before the crashes, according to the documents. People with the disorder are repeatedly awakened and robbed of rest as their airway closes and their breathing stops, leading to dangerous daytime drowsiness.
NJ Transit engineer Thomas Gallagher told investigators he only remembered looking at his watch and the speedometer, blowing the horn and ringing the bell before his packed rush-hour train slammed into Hoboken Terminal at more than double the 10 mph speed limit.
A conductor standing on a platform told investigators he couldn't see the engineer through the cab window as the train rumbled into the station, indicating Gallagher may have slumped down or fallen.
Falling debris from the impact killed a woman standing on a platform. About 110 people aboard the train were hurt.
"The next thing I remember was a loud bang," Gallagher recalled, according to a transcript of his Oct. 1, 2016, interview. "I was getting hit with dust and dirt. I was thrown about the cab. I hit my head, the back of my head, I presume on the wall behind me. And then I had a period where I was going in and out of consciousness."
LIRR engineer Michael Bakalo told investigators he only remembered approaching Atlantic Terminal in Brooklyn and then getting thrown from his seat. He said he wasn't aware of the impending crash. More than 100 people were hurt when the train crashed into a bumping post at the end of the tracks.
The impact launched the lead car into the air and it came to rest on top of the concrete platform.
"At that point I didn't know what the hell was going on," Bakalo said. "I remember being thrown from the seat because I was into the dashboard area, and just, you know, screaming and smoke, and people were laying on the floor in front of me."
(The Associated Press contributed to this report.)