The incident happened on the high-speed Thalys service near Arras on Friday. A 26-year-old Moroccan man was arrested.
One of the Americans said they took an AK-47 assault rifle and a handgun from the attacker as they saw him walk down the aisle of the train.
One of the Americans and another passenger were seriously hurt.
In a press conference on Saturday, French Interior Minister Bernard Cazeneuve said the suspect's identity had yet to be confirmed, but it was believed that he had radical Islamist beliefs.