Alek Skarlatos, a 22-year-old member of the US National Guard from Oregon, said his friend, who is also in the military, had been injured while he grappled with the gunman. Both were in plain clothes at the time.
"I just got back from Afghanistan last month, and this was my vacation from Afghanistan," Skarlatos said.
The 26-year-old gunman was armed with a Kalashnikov automatic weapon. After his arrest he said he was of Moroccan origin. His motives are not yet known.
After the attack, the Thalys train with 554 passengers on board was stopped at the station in Arras, in northern France. All the passengers were interviewed before continuing their journey. French actor Jean-Hugues Anglade, who appeared in the 1986 film "Betty Blue," was on board the train.
French Interior Minister Bernard Cazeneuve went to Arras and commended the Americans: "Without their courage, we would have surely faced a terrible tragedy."
The US White House Press Office said in a statement: "Echoing the statements of French authorities, the president expressed his profound gratitude for the courage and quick thinking of several passengers, including US service members, who selflessly subdued the attacker."
President François Hollande issued a statement Friday night saying that "everything is being done" to determine what had happened on the train. The investigation is being carried out by France's antiterrorism authorities.
The gunman was reported to have boarded the train in Brussels, where luggage does not pass through X-ray machines or other forms of screening. There are often plainclothes security officers on the trains and uniformed officers at the Gare du Nord, where the trains arrive in Paris.