Rachel Watson
The protagonist of the story, she is the title's girl on the train. She goes every day on the same morning and evening trains, observing the same suburban houses beside the tracks. However, there is one house that she watches every day, naming the people in it, Jess and Jason. She loves this house because it reminds her of her past, perfect life, before her divorce from Tom. However, after witnessing a tryst going on between "Jess" and another man, she gets involved in a crime scene related to this house's inhabitants.
Megan Hipwell
Rachel's "Jess," she feels conflicted in her marriage, often seeking comfort in her husband Scott but also finding him overprotective. She has affairs with her therapist and, we later find, with Tom. On a Saturday night in July, she goes missing and days later she is found dead. Much of Megan's trauma comes from the death of her brother Ben and the later death of her child, Libby, who she had while living with a boyfriend named Craig.
Scott Hipwell
Rachel's "Jason," he is suspected of killing Megan because they had a fight prior to her departure and subsequent death. He and Rachel form a friendship and brief sexual relationship when she reveals what she learned about Megan's affair. In the end, he is proven innocent.
Kamal Abdic
Kamal is Megan's, and briefly Rachel's, therapist. He is the man Rachel sees kissing Megan from the train. He and Megan were having an affair, but he suspects it was based on transference, a common occurrence for patients and their therapists. However, he indulged anyway. He is suspected of killing Megan due to this romantic link, but he denies this and is set free due to insufficient evidence.
Tom Watson
Tom is Rachel's ex-husband. Tom and Rachel divorced after Rachel grew depressed by their infertility and he began cheating on her with Anna. Though Rachel still very much loves him two years later, she discovers that he was having an affair with Megan during his marriage to Anna and that he was Megan's killer. Tom is a compulsive liar, which he covers with charm and by blaming others, especially the women in his life.
Anna Watson
Anna is Tom's new wife. She hates Rachel and gets annoyed when Tom talks to or about her. Anna is very happy with her life with Tom and their baby girl, and takes pleasure in power. She especially liked being the "other woman" when Tom and Rachel were still married. However, after finding out that Tom lied to her just as much as to Rachel, she helps Rachel kill Tom, prioritizing her child and herself over him.
Ben
Ben is Megan's brother, though readers hear about him only in her memories. Ben was an exciting and adventurous brother who died at a young age.
Mac (Craig McKenzie)
Mac was Megan's boyfriend when she was fifteen, though he was twenty-two at the time. Megan met Mac when she ran away following her brother's death and they were very much in love for a while. However, after Megan got pregnant, their relationship did not grow with the changing circumstances, and when the baby died, Craig left her. Megan searches online for Mac later in life but cannot locate him.
Cathy
Cathy is Rachel's flatmate. Rachel moved in with her after her divorce from Tom, intending it as a short-term living situation. However, Rachel has lived with Cathy for over two years by the time the story takes place. Cathy is very unhappy with Rachel as a roommate since she often drinks to excess and makes messes.
Damien
Damien is Cathy's boyfriend. He agrees with Cathy that Rachel has a problem with drinking and is not currently fit for a romantic relationship.
Detective Inspector Gaskill
He is one of the detectives working Megan's case. Rachel has a hot and cold relationship with this detective, who sometimes is kind to her and sometimes is more rough in his questioning.
Detective Inspector Riley
He is the second detective working on Megan's case. Riley is generally antagonistic to Rachel, believing she is an unreliable source of information due to her alcoholism and obsession both with her husband and with Megan's case.
Andy (The Red-Haired Man)
Rachel sees the red-haired man often on the train she rides to and from London. He is with her on the night that Megan disappears and later fills her in on some of what she cannot remember. He seems to also have a drinking problem.
