In which novel does the main character become obsessed with a couple she names Jess and Jason?

"The Girl on the Train" is a 2015 psychological thriller novel by British author Paula Hawkins that gives narratives from three different women (Rachel Watson, Anna Boyd/Watson, and Megan Hipwell) about relationship troubles and binge drinking.

Rachel Watson is a 33-year-old alcoholic, reeling from the end of her marriage to Tom, who left her for another woman. Rachel's drinking has caused her to lose her job; she frequently binges and has blackouts.

Rachel follows her old routine of taking the train to London every day, one at 8:04 in the morning and the other at 5:56 in the evening. She begins watching from the train an attractive couple who live a few houses away from Tom. She idealises their life (christening them "Jason" and "Jess"), though she has no idea that their life is far from perfect. The wife of the couple, Megan Hipwell ("Jess"), has a troubled past. She finds her life boring with her husband Scott ("Jason"), and escapes from her troubles by taking a series of lovers.

The film rights were acquired before the book was even published, in 2014, by DreamWorks Pictures for Marc Platt Productions. The American film adaptation, starring Emily Blunt and directed by Tate Taylor, was released on 7 October 2016.