This is a wild ride of a book, reminiscent of both Gone Girl and of Hitchcock. The heroine, Rachel, rides the same train each day, that takes her past the back of her old marital residence. Her ex is now happily married and living in the home with his new wife and baby. Rachel, depressed and alcoholic, has nothing in her life beyond what could have been, until, one day, as the train passes, she sees the young woman who lives several doors down from her old house kissing a man who isn't her husband. Rachel, craving a life, even if it isn't hers, becomes obsessed with the infidelity, and, when the young woman subsequently disappears, she (Rachel) becomes involved in the disappearance in ways she could have never imagined.
This is a page turner, plain and simple, with the same compelling voice and endless twists and turns of Gone Girl. Rachel is as damaged as she is perceptive. We never know if what she sees is real or if what she concludes is realistic. A bonus is that the train, itself, is the perfect vehicle (no pun intended) to show decisions we make when we are both removed from reality and, at the same time, privy to an experience of it that others cannot have.
Get on this train, settle back, and let it take you exactly where it wants to go.