Disassembling the Bachman Reversing Trolley set Engine House

[MLampner]

I want to do some kit bashing with the engine house for the auto reversing trolley. To do do it would be helpful to be able at the least to remove the roof but can not figure out how to do that. I can see there are tabs that fit into slots on the side walls but see no way to release them. I can see inside the house that there are screws that hold it to the base plate but they are inaccessible with the roof on.

Any help would be appreciated.