Some compared it to Alfred Hitchcock's Rear Window, where an ordinary person happens to see a crime. In this case, Rachel is a bit of voyeur who witnesses an act of infidelity.
On her commuter train, she sees a couple whose perfect life she romanticizes. She has recently divorced from her own "perfect" husband. The couple whom she names Jess and Jason becomes her ideal until she witnesses something disturbing.
She comes forward to the police to report what she knows. Sgt. Riley thinks she is a bit of kook. No one takes Rachel seriously because she is an unreliable witness. Rachel has had an alcohol problem even before Tom left her for another woman. Since then, its only gotten worst.
Hawkins gives us the situation and then alternates between many characters' point of view--a difficult juggling act. What is amazing is that none of the tension is lost as she moves from character to character. She withholds just enough to keep the pacing taut and suspenseful.
Psychological fiction and unreliable narrators are hot right now; there are many read-alikes to choose from. This one happens to be one of the best.