I think I may have found some more parts compatibility. I have not confirmed it, but these currently available parts look to be from the same mold as the luggage rack poles from playmobil passenger cars:
Also, I think I found a good candidate for passenger car doors:
The door from this set 5557:
Door: 30229603 Frame: 30229593
The frame could be easily glued outside the doorway with a mild glue without altering the original:
Or if you wanted to sand or file some things a bit, you might be able to make the door fit right into the original doorway:
It seems to also come in green from this set 9126:
Part number: 30052352
I suppose they could be painted black for this application:
With the frame the treehouse door fits the caboose about the same as with the passenger cars:
But without the frame, I think the doors fit even better on the caboose than on the passenger cars. The length and width are absolutely perfect to fit in the caboose doorway(the passenger car doorways seem slightly narrower). All that is needed is a little divit or shallow hole drilled, melted, or filed into the caboose doorway head and doorway sill for the hinge bumps of the door to rest in so it can fit, swivel properly, and remain securely in position without glue. You can see the height is perfect here with only the hinge bumps extending beyond the doorway:
And the width is perfect here:
Here I only meant to show the width. But the gap on top of this last picture shows about how deep the hinge hole needs to be, as the hinge bump is preventing the door from being flush with the doorway head in this picture.
Don't go buying up all of the doors just yet, I still need to get a few more for myself.
Yes, you're quite right, when Playmobil produced the "find spare parts" function it operated with their new computerised system which didn't have the spaces between the groups of part numbers and so if the spaces were included, the part was shown as out of stock. That was a bit of poor software specification and, in my view, shouldn't have happened. Certainly, when I specified software for a new computer system, I made sure it was backwards compatible with existing data layouts.
By contrast, on playmoDB, you can search with or without the spaces in the part number and still find what you want.