How do you get the bottom plate, gear cover off the bottom of Lionel Steam engines (Hogwart's Express) to grease gears??

I have a couple of different engines I have tried to get the bottom plate cover off to check and grease the gears. The Hogwart's 4-6-0 in particular I cannot get off. Attached are photos. There are 6 small screws around the edge and corners on the bottom plate that come out and the ends are then loose but will not come off. There are 2 screws in the middle holding on the center rail pickup rollers they turn but do not undo like there is a bolt on the back of each that is spinning. On the inside there are similar 2 screws holding the wire contacts to the ? Pickup rollers, these as well spin like there is a nut behind that is spinning.

Any idea how to get the bottom plate off I need to change a motor in one engine??