The Concept in 4mm

I described this in a reply to a comment as  the Apa box layout I should have built."

The piece of tissue paper was worried to death by one of our poodles

It would mean compromising the visual side of the model in favour of some operating potential. To make it work real care would be needed in blending in the back scene, and some trickery would be needed to convince the viewer that something actually exists off scene. I thought point rodding could be a good example of that, suggesting that there are actually turnouts at each end of the "loop"

The "ready weathered" Dapol gunpowder van is really dreadful

Is there any point in building it now? I'm not sure it. It is a chance to try out using the loco lift components to build both a traverser and a sector plate. I'm also going to need to try out EM track construction and this would seem a possible practical use of the first usable attempts. A baseboard should take minutes to knock up, and of course I do still have a spare Apa box.

So who knows, it might happen.