Older Proto GP-38-2, besides removing the couplers, there maybe two tiny screws hidden one each under each truck towards the fuel tank. Move the truck(s) in one direction or the other and use a flash lite to help locate the screws. Be careful since these screws like to "Jump" into the trucks or the floor.
Be careful since they may have lighting attached to shell or in my case two lamps at either end of the locomotive attached to the frame.
I just worked on my older Proto GP38-2 and disassembled the locomotive many times in the past month or so.
Greg