Rachel is a mess, she has gone through trauma and has not come up swinging, she fell and can't get back up. Still, she tries to carry on somehow, clinging to the routine of her daily train commute and the escapist fantasy she has projected on her ex-husband's new(-ish) neighbours. When that fantasy is shatterred by real tragedy, Rachel feels compelled to get involved, against everybody's wishes and despite her drunken blackouts. 
 What I loved: all the female characters in the book are flawed but they are not villainized, they are real, imperfect people each struggling in their own ways. 
 What I didn't love so much: the identity of the killer becomes obvious pretty fast, which makes the big plot twist feels totally anti-climactic; surprise!, said no-one. 
 In conclusion: an enjoyable, if predictable, psychological thriller.