On a high-speed Thalys train passing through the Northern French city of Arras, when a French passenger attempted to enter the bathroom, he encountered the gunman and tried to overpower him.
A gun was fired and one passenger was injured.
The gunman was carrying a Kalashnikov rifle, an automatic pistol with ammunition clips, and a box cutter knife, Mr Cazeneuve said.
American Spencer Stone seized the gunman, while Alek Skarlatos grabbed his guns, according to accounts from the passengers.
A friend of theirs, Anthony Sadler, and Chris Norman, a British man who lives in France, also helped restrain the attacker.
Mr. Stone and Mr. Skarlatos are members of the Air Force and the National Guard respectively.
President Obama has called the three men to commend their “extraordinary bravery”.
US Secretary of Defense Ash Carter praised the three, saying that the two servicemen had shown why “on duty and off, ours is the finest fighting force the world has ever known”.
French President Francois Hollande phoned Obama to thank him for the “exemplary conduct” shown by the men.
Prime Minister David Cameron also praised their “extraordinary courage”.
The suspect was detained at the train platform.