The Commuter

Michael MacCauley, a former police officer, is a family man with a wife and son. He goes through the same routine train commute to work and back every day, only to be fired from his job as a life insurance salesman after ten years.

On the train, Michael meets a mysterious woman named Joanna. Joanna proposes a hypothetical situation to Michael, asking him to do one little thing, the results of which would have consequences that he would know nothing about, but would affect one of the passengers on the train. Joanna's one little thing that she wants Michael to do is locate "Prynne," an unknown passenger, and plant a GPS tracker on them. Out of curiosity, Michael searches the bathroom and finds an envelope with $25,000, and realizes that the situation is not hypothetical. Read more on Wikipedia.