From USA Today:

The French newspaper La Voix du Nord reports that the gunman was overpowered by passengers, including the two Marines. The newspaper said the pair heard the man loading the automatic weapon in a bathroom on the train and overpowered the gunman before he could open fire inside the main train cars. Earlier reports said that one of the soldiers was British.

“As he was cocking it to shoot it, Alek just yells, ‘Spencer, go!’ And Spencer runs down the aisle,” Sadler told AP. “Spencer makes first contact, he tackles the guy, Alek wrestles the gun away from him, and the gunman pulls out a box cutter and slices Spencer a few times. And the three of us beat him until he was unconscious.”