The alleged attacker, 25-year-old Moroccan national Ayoub El Khazzani, boarded a high-speed train in Brussels bound for Paris on Friday carrying a Kalashnikov rifle, an automatic pistol, ammunition and a box-cutter.
Witnesses say he opened fire, injuring a man before being wrestled to the floor and overpowered by Spencer Stone, Alek Skarlatos and Anthony Sadler, who have been hailed as "heroes."
"We had first-class tickets to the carriage where the incident took place. We were sitting in a different carriage because we couldn't find it at first," said Sadler, a 23-year-old student.
"We decided to get up because the wi-fi was not good... We decided to go to the car about a half hour into the train ride," he said.
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Stone, who was injured by the gunman and discharged from hospital yesterday, said he had just woken up from a deep sleep when the alleged attacker emerged.
"It looked jammed or it wasn't working and he was trying to charge the weapon," added Stone, who serves in the US Air Force.
"Alek hit me and said 'let's go'.
"We tackled him and hit the ground and Alek grabbed the gun while I put him in a choke hold. He kept pulling more weapons left and right. He pulled out a hand gun and a box cutter and started jabbing at me with that," he said.
"The guy had a lot of ammo. His intentions were pretty clear," said Skarlatos, a 22-year-old member of the National Guard in Oregon who was on holiday in Europe with his friends after he had recently returned from Afghanistan, adding the gunman "never said a word."
For Sadler, "hiding or sitting back" was never going to accomplish anything.
"The gunman would have been successful if my friend Spencer had not gotten up. I want that lesson to be learned. In times of terror like that to please do something. Don't just stand by and watch," he added.