I model the C&O and as such Flat ground was rare on the prototype.  While the area I model (the New River Gorge) was flatter then other area it still had a bit of a grade.  And while I flattened out major switching areas I do have a few Coal tipples and such where the train being switch will have to sit at least partly on an incline.  Part of this is due to geometry.  One of my tipples had a track going up from the main to the tipple and back down to the main.  So part of the train will be on an incline.