We spend over half the book with Rachel Watson, our titular girl on the train. She's not your typical protagonist, though. Describing herself, she says:
I'm not beautiful, and I can't have kids, so what does that make me? Worthless. (7.119)
Yeah, her self-esteem is at an all-time low, so she's not just a girl on a train, she's an unemployed drunk with an "overactive imagination" (1.1) on the train. Or, "barren, divorced, soon-to-be-homeless alcoholic" (5.2) on the train. Or, if you were to ask Anna or Megan, our other two point of view characters, she's a creepy, desperate, stalker, weirdo on the train. How do we rationalize all these different angles of Rachel's character into one person?
Rachel is trying to do the same thing we're attempting to, which boils down to trying to rationalize drunk Rachel with sober Rachel. We know what drunk Rachel can do—get angry, violent, and destructive, then black out to the point that she doesn't even remember what she does. But we never get a good idea of what sober Rachel can do. She's analytical and determined, but unfortunately she's overshadowed by her chronic drinking problem.
Rachel's journey is about trying to find herself, as trite as that may seem. What keeps it interesting is her battle against addiction along the way. Sober Rachel takes one or two steps forward, but drunk Rachel pulls her one or two hundred steps back. Sober Rachel wants to be helpful and figure out what happened to the missing woman, Megan Hipwell. Drunk Rachel is self-destructive, groveling after her ex-husband, Tom, despite the fact that he is remarried to Anna, the woman he cheated on Rachel with.
The greatest accomplishment for sober Rachel is to be useful, to have a purpose. Sometimes that purpose is as simple as being a listening ear. When Megan's husband, Scott, relies on Rachel for comfort, she says, "I've become a sounding board, and I'm glad of it. I'm glad to be of use to him" (11.101). She doesn't care that she's just an object and Scott would be just as happy talking to basically anyone else; Rachel needs to find a greater purpose in order to be content and to stop drinking.
Okay, Rachel isn't quite Dr. Jekyll and Ms. Hyde, but she does possess two distinct personalities. Or does she? Like the subject of Stevenson's classic story, sober Rachel is consumed with guilt over the horrors enacted by her evil alter ego. She has bad dreams and wallows in self-pity. On top of that, she feels bad that she doesn't feel bad enough, because during her blackouts she can't remember transforming into Ms. Hyde.
She doesn't remember… because she never really turns into Ms. Hyde. She only thinks she does. But as Rachel learns, her violent outbursts, like hitting her husband with a golf club, never happened. Her emotionally abusive husband, Tom, took advantage of Rachel's blackouts to manipulate her and tell her she did things she never actually did. She didn't hit Tom with a golf club; he attacked her. And then he had the gall to taunt her: "What happened to you, Rachel. […] When did you become so weak?" (7.208). We have an answer: When Tom made her that way.
Rachel uncovers the deception by being introspective and analyzing her feelings. In other words, sober Rachel cracks drunk Rachel's case. During the golf club incident, for example, she didn't feel anger and rage—she felt fear. This is empowering for Rachel, and it enables her to tackle the blackout she had the night of Megan's disappearance. When she does, she uncovers fear (of Tom) and a memory that Tom got into a car with Megan.
And so Rachel realizes that Tom, always the philanderer, was having an affair with Megan and killed her. Rachel theoretically kills two birds with one stone here. She solves Megan's murder and she absolves herself of the guilt accumulated from her relationship with Tom. This dual accomplishment inspires Rachel to sober up and start anew. We last see Rachel on a vacation to clear her mind. Where do you think she'll go the next time she gets on a train?