"I'm just a college student, it's my last year in college, I came to see my friends on my first trip in Europe and we stopped a terrorist, it's kind of crazy," joked one of the trio, US student Anthony Sadler.
Chris Norman, a 62 year-old British consultant who lives in France, told reporters he had been sitting in the same carriage as the Americans when they heard a shot.
"I looked up and saw a guy carrying an AK-47 or at least I assume it was some kind of machine-gun," he said.
Mr Norman explained that the men eventually got the gunman under control.
"It could have been a real carnage, there's no question about that," he added.
Alek Skarlatos, 22, a member of the US National Guard from Oregon confirmed his friend, called Spencer Stone and also in the military, had been injured while he grappled with the man.
"Spencer ran a good ten metres to get to the guy and we didn't that his gun not working or anything like that, Spencer just ran anyway and if anybody would have gotten shot it would have been Spencer for sure and we're very lucky that nobody got killed, especially Spencer," said Mr Skarlatos.
Mr Skarlatos added that he had just returned from Afghanistan and was also on his first European holiday.
The men said Spencer was in good spirits and that he was "in disbelief" over the incident.
Three people have been injured in the incident, including the US serviceman and another unidentified passenger.
The three men also showed off their bravery medals which had been given to them by the local mayor for their actions.