It could have been absolute carnage, no where to get to safety for any of the passengers, it doesn't bear thinking about the potential horror of it.
Luckily the Marine noticed the terrorist acting suspiciously, not many people would recognise the sound of a gun being loaded for behind a locked door. Those 4 guys were so brave to tackle someone with an AK47, he could have even been a suicide bomber.
Interestingly, I was taking a train from Lyon to Switzerland last week and there were 4 heavily armed security patrolling the platform, not sure if they were police or army, but first time I have ever seen that. Wonder if they already had an inkling of a terrorist threat on a train.