Everything that plays with unreliable narration and somewhat fancy structure that features a woman protagonist is likely to be compared to Gillian Flynn's Gone Girl. Really it's unfair, but it's hard to resist comparisons to a book that made such a huge impact in the recent past. So, acknowledging the unfairness, I can't help but think about The Girl on the Train in the shadow of Gone Girl.
Although I was curious about what happened to Megan and who was responsible, I was less than compelled because I didn't come to care about her until the last third of the book (beyond, you know, she's a human being and killing or abducting one is wrong), and then not much. No one's likable in this book, and I can't explain why sometimes I love and am drawn to unlikable characters, and sometimes I just plain don't like or care about them. Voice is likely a factor; Amy's voice in GG is sharp and charismatic. Rachel, Megan, and Anna's voices feel familiar and not particularly striking.
I appreciate that by the end it turns out Tom, Rachel's ex, Anna's current husband, and Megan's former lover (and father of her unborn child) is the guilty party in more ways than one. He not only cheated on two wives and killed Megan, he made both Rachel and Anna feel responsible for things which may or may not have happened. By book's end, Anna and Rachel aren't exactly allies despite their shared traumas. Despite this, the book's not saying anything terribly original about gender. One woman accidentally let her baby drown (and later became pregnant again and vowed to do it right this time), one is a mother of a baby, and another's life fell apart because she couldn't have children. Having or not having (and being capable of taking care of) children defines these characters and drives the narrative in subtle ways.
Mysteries and thrillers are not my favorite genre, though when one hits, it hits (like GG). Still, I occasionally try one that seems less beholden to its genre, and I think The Girl on the Train sticks to the form more than I prefer.