This is an unreliable narrator story, with multiple twists revealing that all is not what it seems, again and again.
Rachel is divorced, bitter and alcoholic. Her husband is now married to Anna, and they have a child together. Rachel goes on a train everyday, and at a specific spot where the train stops for a few minutes, she observes a seemingly happy perfect couple. She slowly becomes obsessed with the couple, giving them fictitious names and lives. The woman Rachel observed through the train window is Megan.
The story shifts perspectives, describing events from all three viewpoints, Rachel, Anna and Megan. Without revealing too much, Megan goes missing, and Rachel inserts herself into the investigation, but her alcohol fueled blackouts hide a lot, both from her and the reader.
A great, fast paced read. I first struggled to get through this book because I generally do not like unreliable narrator tomes, but after a long break, I gave it another go and rather enjoyed it.