Authorities praised two U.S. military members and their friend who tackled and subdued a man armed with guns and a box cutter on a Paris-bound train Friday as it sped through Belgium, breaking up what could have been a deadly terrorist attack.

The three Americans were seated on the train when they heard a gunshot and breaking glass, according to accounts from one of the men and a U.S. official briefed on the attack.

Crouching...