The Girl on the Train is a decently paced, entertaining & an adequately well crafted thriller.
The first person storytelling where the principal narrator is an unlikeable human being; someone who makes your insides squirm & yet makes you root for them - is difficult to pull off and I think this book has managed to do that fairly well.
The premise is different and original - a depressed, alcoholic woman - who on her way to work via train everyday - likes to watch and obsess over a seemingly perfect couple living close to the train tracks - doing their household, routine things. The story starts from this innocuous fascination of this woman with the couple.
But then again, it becomes too messy and melodramatic after a while. You sort of figure out who the killer is and the characters take such an unnecessarily long & winded path to get to the conclusion that you wish there was a way to fast-forward the whole thing. To be honest, even though the mystery is good - I found the execution of the ending just meh.
In trying to make every character a complex, hot mess, some of them have actually turned out to be pathetic & pitiable.
Overall, I would give it a 3/5.
Give it a try with realistic expectations and turning a blind eye to all the hype (this was apparently rated as one of top books of 2015).