Hello I have not read the book but have heard its a good story. If I may make the following corrections to your very good writing.

With "on the train" the meaning gives the impression she is riding on the train with it moving. 

I would not say "The girl in the train" I would more think to say "The girl sitting in the train" which could be moving or stationary. So its a subtle effect. I have the same trouble with learning Dutch with op, om etc.

The girl on the train

I read the book “The girl on the train”. Have you read it too ? ok you could say Have you also read it?
Could we also say “The girl in the train” ? Is there a difference ?
Anyway, it is about a women who takes a train every day. On the journey she that and uses to watch sees a nice (or beautiful) house with a nice couple by from the train's window. She can't wait everyday to watch see them. She imagined the perfect life they must have and that she hasn’t.
One day, she learnt in from a newspaper article  that the women is missing. The girl on the train is very interesteding by the case and so she starts to collect information about the couple to find out what happened.

I prefer to use see not watch as watch has the sense of looking for a long time where on a train I assume you will be moving fast so you have a short time to see the house and couple.

Hope I have not confused you.