Rachel is a 32-year-old divorced woman who commutes daily to London, maintaining the facade of going to a public relations job she has actually lost. She lives with a flatmate and struggles with severe alcohol use disorder and memory blackouts following the dissolution of her marriage. Deeply lonely, she projects an idealized narrative onto a couple she observes from the train window every day.

Megan is a young woman living with her husband in the suburbs along the train tracks. Though she appears to have a perfect life from the outside, she experiences intense restlessness, insomnia, and panic attacks. She has a history of trauma and frequently changes jobs, struggling to find contentment in her domestic routine.

Anna is Tom's new wife and the mother of their baby, Evie. She formerly worked as an estate agent before settling into suburban motherhood. She feels intensely protective of her family and is highly distressed by Rachel's continued presence and phone calls, wanting nothing more than to be left alone to enjoy her domestic life.

Tom is Rachel's ex-husband and Anna's current husband. He lives with Anna and their daughter Evie in the house he previously shared with Rachel. He presents himself as a patient but frustrated man trying to manage his ex-wife's drunken boundary-crossing while maintaining his new family life.

Scott is Megan's husband, working in IT. Rachel observes him from the train and idealizes him as "Jason," half of a perfect couple. In reality, he is intensely devoted to Megan but struggles with possessiveness, monitoring her communications and pushing her to attend therapy to fix her unhappiness.

Kamal is a Bosnian refugee and therapist practicing near the suburbs. He treats Megan for her panic attacks and insomnia, offering her a calm and reassuring presence. His relationship with his patient becomes complicated as boundaries begin to blur during their sessions.

Cathy is Rachel's friend from university and current flatmate. She provides Rachel with a place to live after her divorce but grows increasingly exhausted by Rachel's drinking, messiness, and failure to pay rent on time. She operates under the false assumption that Rachel is still employed.

Gaskill is a police detective investigating Megan's disappearance. He is methodical and initially seems more receptive to listening to Rachel than his partner, though he questions the reliability of her statements due to her drinking habits.

Riley is a police detective working on the missing persons case. She is sharp, direct, and openly skeptical of Rachel, quickly noting the inconsistencies in Rachel's stories and her history of erratic behavior.

Andy is a commuter with red hair who regularly rides the same train as Rachel. He notices her drinking and her distressed state, occasionally offering small gestures of politeness or assistance during her more difficult moments on the journey.

Evie is the infant daughter of Tom and Anna. She represents the perfect family life that Anna fiercely protects and Rachel desperately mourns not having.