If you chose to read this book, set aside a day because you will not want to put it down. It has elements similar to the widely popular GONE GIRL. Naturally when Megan disappears, the husband is a prime suspect and there are plenty of clues that he MAY be controlling and abusive.
When Megan herself begins to narrate some of the chapters, readers find she is complicated, reckless and with a past she tries to keep hidden. Could she have chosen to disappear? What is she keeping from her husband and others? Similar to my reaction to both the husband and wife in GONE GIRL, I kept switching from suspecting one spouse to the other, and the more background info I learned, the more suspicious I was of everyone. At times I even suspected Rachel, even though she kept professing never having met the couple. In the end, I did guess most of the ending, but I would have been just as convinced if the book had ended in another way.
Rachel's viewpoint as a voyeur reminded me of the classic Hitchcock movie REAR WINDOW in which a wheelchair bound lawyer believes he has witnessed a murder, but no one will believe him. Like that invalid, Rachel is not believed; and as she continues to act erratically, she is viewed as less and less reliable, even by readers. I loved the author's use of the train -- for so many people, it is an unavoidable pattern to their day, something that uses time but adds nothing of value to their lives. To Rachel, it was the last cord to a life lost. If she gave up being THE GIRL ON THE TRAIN, she would finally lose herself.