Informal 'what would people do if...?' types of studies on the matter have shown that people do, indeed, often just sit by and turn away when a crime is happening in front of them because they're too scared or sometimes just too apathetic (but usually scared) to say or do something. Sometimes there's this attitude of, 'It's not my business' or, 'What could I possibly do to help?' I don't honestly have any idea what I'd do if I saw something like this--I like to think I'd help, but I just don't know. I'm certainly not an intimidating person, so aside from calling for help I have no idea what I COULD do, apart from make one victim into two by doing something rash and very stupid.
As for being on a crowded train--rapes happen everywhere, and to be honest a crowded train isn't as crazy a place to do it as you'd think. People all mashed up together like sardines with no breathing room... if an attacker gets a victim in the right place, they have little hope of getting away and people nearby may even think that they can't possibly be witnessing THAT and must instead just be imagining things.
Or it could be a place like India, which has such an aggressively terrible collective cultural attitude about rape that even high-ranking police officers frequently believe that women bring rape upon themselves for a variety of victim-blaming reasons ('SHE DRESSES LIKE A WHORE!' 'SHE'S HANGING OUT WITH MEN!' 'SHE DOESN'T HAVE A CHAPERONE!') and that only a handful of reported rapes are 'genuine' and that a woman who reports a rape deserves all the hate and criticism and scrutiny she gets. If I were an Indian woman, I'd probably figure I had nothing to lose jumping from a train to escape a rapist.
