Off-duty US airman Spencer Stone said he had just woken from a deep sleep when he saw the gunman and moved to restrain him on Friday.
His friend, Alek Skarlatos, a member of the US National Guard, said the gunman appeared to have no training.
French authorities say the suspect, Moroccan national Ayoub El-Khazzani, 25, has links to radical Islamism.
French police are questioning the gunman who was flagged up to French authorities by their Spanish counterparts in February 2014.
He is reported to have lived in France, Spain, and Belgium and to have travelled to Syria.
Mr Stone, Mr Skarlatos and a third American, Anthony Sadler, have been hailed as heroes for their actions on Friday, and were giving a press conference at the US embassy in Paris.
It was the first appearance before the cameras for Mr Stone, who has been treated in hospital and was wearing a sling because of injuries to his thumb. Asked what led him to react, he replied “survival”.
“I turned around and I saw he had what looked to be an AK-47 and it looked like it was jammed or wasn’t working and he was trying to charge the weapon.
“Alek just hit me on the shoulder and said ‘let’s go’ and ran down, tackled him. We hit the ground.”
Friday’s incident began when a French passenger tried to enter a toilet, encountered the gunman and tried to overpower him. It is thought this passenger may have since requested anonymity.
A gun was fired and a French-American passenger was injured by the bullet. Mr Stone said he saw blood squirting out of his neck.
“I just stuck two of my fingers in the hole, found what I thought to be the artery, pushed down and the bleeding stopped,” he said. “I just said ‘thank God’ and held that position until the paramedics got there.”
Mr Skarlatos also said his initial reaction was “mostly just gut instinct”, and that military training had only played a role in providing medical help and making sure there were no accomplices.
Mr Skarlatos said the gunman had a lot of ammunition and “his intentions were pretty clear”, but that he appeared untrained.
“He clearly had no firearms training whatsoever.”
“If he knew what he was doing, or even just got lucky… we would have all been in trouble and probably wouldn’t be here today – along with a lot of other people.”
Mr Stone said Friday’s events had still not sunk in. “It feels very unreal. It feels like a dream.”
French President Francois Hollande is due to present the three Americans, and Briton Chris Norman, with the Legion d’Honneur on Monday, in recognition of their bravery. Mr Norman also helped restrain the gunman.
Security aboard the high-speed Thalys service on which the incident took place is being stepped up. The trains link major cities in the Netherlands and Belgium to Paris.
Patrols and security checks will also be boosted at international train stations, and more baggage checks will be carried out.
France’s security services have placed been on high alert since January when Islamist militants killed 17 people in and around Paris – including the attacks at the offices of satirical paper Charlie Hebdo and a Jewish supermarket.