"Girl" ? Seriously?

Girl seems to be the politer female version of guy, which is "gal". "The gal on the train" doesn't really sound good, nor does "The lady on the train", as Lady is the female version of "gentleman". So we're left with either the "woman on the train" or "the girl on the train". Can you see why they chose girl? The woman on the train almost sounds like "The women on the train"... again, not a great sound.

Stories like Girl on the Train and Gone Girl (both written by women, so patronizing is a false accusation) deliberately employ the word "girl" to create a sense of innocence. It disarms the reader. We immediately see these "girls" as innocent, young, and naive just by reading that word in the title, and that little word delivers to us free sympathy for the character without reading a page, but as we read these books, these "girls" prove to be not so innocent and not so naive and that they are in fact not victims. In the end, they prove that they are, in fact, women, not girls.

"There are too many of them. Can't kill the world."

Girl = Boy
Girl = Guy
Woman = Man

the problem is that there's not a good female equivalent for Guys and that you're comparing girl to boy instead of guy

I had forgotten the exact name of the movie. I search The Woman On The Train and The Lady On The Train knowing it's about an adult woman and it doesn't show up. So I see what your saying.

Only 1 word off and the title doesn't show up in the search at all. I type in just the word Train and it shows up right away, crazy.

Well, 'Bad Boys' isn't about two children operating as undercover police officers, so I guess it works both ways.

Haven't seen the movie, but have read the book. Perhaps the word "girl" was used deliberately because the author wanted to emphasise the childish way in which she behaved. It's several months since I read this, but my main emotion was annoyance at how adolescent her behaviour was.

STOP MANSPLAINING!!!!! GIRLS CAN DO ANYTHING MEN CAN AND MORE1!!

I'm a woman and I prefer to refer to myself as a girl. As other posters pointed out, woman sounds old. It's like how most women prefer to be called out by 'Miss' rather than 'Madam'. In general women don't want to appear like old things. So don't be so surprised a lot of women prefer to be called girls (the opposite of 'guys', not 'boys').