Author Paula Hawkins Talks Success of ‘The Girl on the Train’

“The Girl on the Train” is shattering book sales records.

— -- “The Girl on the Train” is shattering records this summer as the longest-running number one best-seller.

Good Morning America” caught up with Hawkins in her hometown of London. She said she never expected this level of success.

"Nothing to this extent," Hawkins said.

The runaway hit took the former financial journalist a year to write.

The story follows Rachel, a young, alcoholic divorcee who fantasizes on her daily commute and watches a young couple from the train.

“I used to do that thing – looking in the houses that the train passed and wondering what was going on there, and hoping I would see something interesting and of course I never did. But I imagined what might be going on,” Hawkins said.

Rachel’s world is thrown into turmoil when she sees something terrible, and the novel follows her as she spirals out of control. The ending is shocking.

"There was lots of little bits I played around with," Hawkins said. "Initially there was a different killer, very early on."

While the novel is set just outside of London, the movie will take place in upstate New York. Hawkins said she doesn’t mind the change.

“I’ve spoken with producers and directors and I’m really happy with what they told me,” she said.