How 'bout those United States Marines!

[Gman]

One of the reports I have read indicated they heard the terrorist charging the weapon in the bathroom.. Even through walls a bolt slamming home on a weapon is unmistakable. I am sounds like they stopped them as he was steping out of the crapper to start shooting.

One of the articles stated that the Marines saw this guy looking suspicious and followed him onto the train.

ETA: Quote from the article linked at the beginning of this thread;

Belgian journalist Marin Buxant Tweeted that the US Marines were on leave in Brussels when they spotted the man and followed him on the train. When the suspect went into the toilet, the Marines recognised the sound of a weapon being armed and decided to act immediately.