I agree that what happened is horrific.
However, if you were sitting on that train next to the woman and 6 thugs armed with knives attacked her, what are the chances that you could fight them all off of her w/o getting yourself killed? Maybe if you were a navy seal, or if you had 2-3 of your mates there to back you up.
It is very hard to judge those other passengers w/o knowing the situation; how many other passengers, how many other women and young children, etc.