i was working on something similiar to this a while back, i never managed to make it work, because any thing i did, made terragen react to the overlay as though it were a heightfield, worked fine for trying to use it in motocross madness, but for flat tracks, id say the easiest method would be to create a track in photoshop (making sure the entire track stays the same shade) and just use that as an overlay… world machine might handle overlays better than TG does, im not sure, never tried it in world machine…lol