I'm not sure there's a hard fast rule..........Normally weeds and such grow up thru the ballast, which would dictate laying ballast, then scenic material., But on the other hand, most of the scenery was there before the railroad came through, so it should be scenery, then ballast. If the railroad has done trackwork recently, there would be no weeds. On branch lines and especially sidings, they might be quite overgrown. Personally, I lay the ballast first, that way I can add weeds and such if I want, or leave it clean.