I discovered this tip by accident while masking two box cars in preparation to paint the undersides. I needed to somehow cradle these cars so that they could rest upside down without damaging the vertical brake staff and wheel which rise above the roof line. However, the tape was enough to provide a stable support for the car and clearance for the brake wheel.
The first car to be masked was a Central Valley 36' car (shown above in the bottom of the photo). These cars are short in height, so the Frog tape I used was wide (tall) enough to cover the whole side plus extend a few scale feet above the roof. When I turned the car over, set it on the tape, and realized how well that would work, I masked the second car, a Silver Streak box car, with a full width of Frog tape plus a half width to create enough height to do the same.
Voila! Masks that protect the car sides and roof AND serve as stands for painting. I imagine this would work just as well if you were painting the roof a different color - just extend the tape down the sides below the underbody detail.