I played the Switcheroo scenario. After the second consist is built, You need to decouple the train, which consist of 10 autoracks and 50 of another kind. The moment I uncoupled (using camera 3), the train derailed and the scenario ends. I coupled at a speed of less than 2mph and applied the train brakes a little to hold the train. EDIT: i played again, and this time no issue.
