If I recall correctly its a 4 tab clip on top that needs all 4 tab holes in the top cap to be slightly pried away from the truck. Usually doing one side then the other is the easiest as you only have to deal with 2 tabs at one time. But it can re-clip back on as you endeavor to undo the other side, so a bit of card paper shoved in there works a treat. But its been a long time since I owned a Bachmann .................