The actress will be working with Woody Harrelson, Kate Mara, Eduardo Noriega and Ben Kingsley in the film.
Mortimer will be replacing Samantha Morton in the role, as Morton was forced to back out of the project due to injury after the ceiling in her London home collapsed.
Transsiberian will began filming last week in Lithuania, and will also be shot on location in Beijing and Russia.
The film follows the story of an American couple, Roy (Harrelson) and Jessie (Mortimer), who take a trip on the famous train from China to Moscow. When they meet another couple, Abby (Mara) and Carlos (Noriega), they become embroiled in a murder scandal and investigation.
Ben Kingsley will play a Russian police officer investigating the two couples.