Third-Person (Limited)
The Girl on the Train is actually about three girls—well, three women. There's Rachel, the girl on the train; Megan, the girl being looked at by the girl on the train; and Anna, the girl who stole the husband of the girl on the train.
The book follows a chapter structure of Rachel, Megan/Anna, Rachel, Megan/Anna, and so on, with Rachel being every odd-numbered chapter for the majority of the book. This is fitting for the structure because Rachel is the odd-woman out. And while Megan and Anna are neighbors, Rachel is the outsider—but she's still the one who brings everything together.
Chapter 34 is the first even-numbered chapter Rachel gets, and it happens because Chapters 32 and 33 are about Anna and Megan respectively. This is the moment where Rachel has figured out who killed Megan and firmly inserts herself into the story. She's no longer an outsider; she finally has the power to influence the outcome. She's standing on even ground, ready to take charge.