So to start off, the way the track system works prevents it from scuffing the bag. The tracks are what touch the stairs and the tracks rotate as you move up and down the stairs. Also, the track system slightly protrudes from the back of the bag, so if you were to lay the bag down on the ground, it would be the tracks that made contact and not the bag itself.
Take a look at this 3D rendering - https://drive.google.com/open?id=0Bwcs_Kj5ygHtVHo1cWtHb0NzeUU
In the image you will see the tracks and corresponding glides are slightly raised from the back of the bag.