That kind of attitude leads to inaction, which is worse than doing something. The bystander effect kills.
Sadly, in the previous comments a hypothetical fight/murderer was mentioned as an on-board scenario. Sadly, more than 90% of people will just run away or ignore such a situation. A fair number of people here said that they would evacuate the train, which is all fine and good, except for the people who have been cornered by or already slain by, Amtrak Johnny.
The somewhat recent Greyhound bus murder, where the asian guy used a "rambo knife" to kill a seatmate, and out of a busload of witnesses, no one tried to stop him. All the passengers and driver fled the train, and then were praised as being "heroic" for leaving the train and later disabling and barricading it to lock the murderer in. During this time, the nutcase severed the head of his former seatmate and ran around with it, taunting police and passengers outside.
"Evil will prevail when good does not stand against it"
Sometimes, I wonder how police don't have more "accidental" shootings when apprehending suspects. You're running around with a knife 1/4th your height, wailing like a banshee, and carrying around a head, so we'll just arrest you and shove you in a jail with climate control, 3 free meals a day, a free gym, and HBO? I don't think so, though I already know that I wouldn't make a good police officer.
Again, the people who cut and ran were lauded, and a guy was hacked to death right in front of them. When "heroes" set new land speed records in fleeing from trouble that they could have easily stopped, my hope for the world goes down the drain.