She joins an ensemble that already includes Emily Blunt, Rebecca Ferguson, Haley Bennett, Jared Leto and Justin Theroux with Tate Taylor directing.
The story follows a woman (Blunt) devastated by her recent divorce who spends her commute fantasizing about a seemingly perfect married couple who live in a house that her train passes every day. Along the way, she becomes entangled in a mystery revolving around the couple.
Janney will play a police detective in the pic.
Erin Cressida Wilson penned the script. Marc Platt will produce the film, while Jared LeBoff of Marc Platt Prods. will serve as executive producer along with Celia Costas.
The studio recently dated the film for Oct. 7, 2016.
Janney and Taylor have a longstanding relationship, with the actress not only appearing in his recent hits such as “The Help” and “Get on Up” but also his smaller projects such as “Chicken Party” and “Pretty Ugly People.”
She can still be seen on the CBS comedy “Mom,” which recently won her an Emmy for best supporting actress in a comedy. On the film side, Janney was most recently seen in Fox’s “Spy.” She is repped by Thruline, Gersh and Nelson, Davis.