The story revolves around an alcoholic divorcee woman, Rachel, who becomes obsessed with a couple living in a house she passes on her daily commute into Manhattan. One day during these daiy voyeuristic commutes Rachel witnesses the woman kissing a man who is not her husband. Having been through a divorce with an unfaithful husband - who lives in their jointly purchased house a few doors down from the couple she obsesses about - Rachel loses it.
Rachel, known for making unannounced visits to her ex-husband's home, subsequently tries to visit the apparently unfaithful woman (Megan) to give her a piece of her mind. Unfortunately for Rachel as she was in her usual drunken stupor she has little recollection of what transpired during that fateful visit. Unfortunate because that night Megan mysteriously disappeared.
The story slowly unravels and the viewer pieces together the puzzle, all the while learning more about each character. (Yes, you know the old adage: don't judge a book by its cover.)
The Girl on the Train is billed as a psychological thriller on many move sites. That's a fallacy. At best it's a mystery - albeit incuding a side role for a mysterious psychiatrist. Only the strength of the acting held it together with great performances put in by the cast. In case anyone's asking, I would say the book might be the better bet!