After spending some time in the track editor, I realized how tedious it was to fill in the remaining tiles after I had created my track, especially with a big level. I would suggest an option to convert all the 'clear' tiles into grass/sand.
That already works by settings the tile type while pressing CTRL. I thought that it was in the menu, but it isn't :)
Ah I see now, I'll be using that a lot then :)