In TRS19, there's an option in the Trainz settings. While in Trainz, not at the Launcher, click on the three-bars to bring up Trainz-settings. I can't remember which menu it's on but the option is an easy click on a check-box instead of requiring an edit to the Trainz-options.txt file as in the past.
Once checked, you should be able to use the arrows on your keyboard to move yourself around the cab and even into the attached train cars.