THE GIRL ON THE TRAIN is the story of a troubled alcoholic who gets involved in the murder of a girl whom she hardly knows. Mira Kapoor (Parineeti Chopra), based in London, is happily married to Shekhar (Avinash Tiwary). Mira takes up the case of an African man who gets killed in a shootout. She then receives threats from the family of the accused, Jimmy Baga. Yet, she proves in the court that Jimmy is the killer. Jimmy Baga is sent to jail. The same day, Mira finds out that she is pregnant. 6 months later, Mira is in a happy space as motherhood is something she deeply aspired for.
Unfortunately, Mira and Shekhar get involved in a car accident. Mira suffers from a miscarriage. Her doctor tells that she is not fit to conceive again. Mira is shattered and to cope up with the loss, she resorts to drinking. She drinks so much that she would get violent and suffer from blackouts. And then the next day, she would not remember a thing. During her drunken stupors, Mira ends up assaulting Shekhar and also insulting his boss due to which Shekhar is fired. Shekhar finally divorces Mira. Mira loses her job as well. With nothing else to do in life, she begins to travel in the train from London to it's suburbs and back. On the way, she crosses the place where her house with Shekhar is situated.
However, just next to Shekhar's house, Mira spots Nusrat John (Aditi Rao Hydari). Mira starts observing her whenever her train passes from near her house. Mira sees that Nusrat is having a lovely married life with Anand Joshi (Shamaun Ahmed). She also sees Nusrat dancing without a care in the world. Mira wishes if she had a life like Nusrat. However, one day, Mira sees that Nusrat is hugging someone who is not her husband. The way Nusrat hugs makes her realize that she's having an affair. Mira is shattered. The perfect image of Nusrat which she had crafted in her mind is broken and that angers her. She feels that she shouldn't cheat on her husband as she knows the pain. This is because Shekhar also started an affair, with Anjali (Natasha Benton), with whom he's happily married now. Mira hence decides to teach a lesson to Nusrat. She goes to the latter's house but the house is locked. She then spots her in a forest nearby. Mira charges at her and then blacks out. Mira wakes up at her house with a wound on her forehead and no memory of last night.
The next day, Nusrat goes missing. Officer Kaur (Kirti Kulhari) comes on board to investigate. During the course of her investigation, she finds out that Mira had come angrily to meet Nusrat and hence, Mira becomes the prime suspect. A few days later, Nusrat's dead body is found in the same forest. What happens next forms the rest of the story.