a marine who is trained is never self directed. if the appearance of self direction, he is under general orders, always, and accountable to the chain of command. a marine would never utilize their "training" without being under the orders of the chain of command.
A MARINE DOES NOT KNOW HOW TO DIRECT HIS OWN ACTIONS, BUT WILL BE UNDER THE ORDERS OF THE CHAIN OF COMMAND.
so, if a trained, term-served marine is on a train and utilizes his military training, he does not know how. in the marines he is an element and the commander is the war fighter. now, having kick-ass dudes is a commander's dream, he is, none-the-less, the fighter with HIS elements, as a marine with his standard issue rifle, shovel, canteen, tattoos, etc.
everyone on the train was supposed to have acted out of fear. now, a marine never acts out of fear, but out of obedience to his chain of command, with full effort and esprit de corps, not fear.
the fact the marine didn't know when to let go of the choke hold demonstrates that the devil dog needs his handler, since he didn't know what a leader of his command element would do, he didn't do it.