a cool modelling thought
Now this is a classic. A narrow gauge box car on a standard gauge flat car on a narrow gauge transporter wagon. http://www.bahnbilder.de/name/einzelbild/number/30764/kategorie/Deutschland~Schmalspurbahnen~Wei%DFeritztalbahn.html John

A great space saving idea. Could easily be applied to model railways as well.

Could we possibly expand this idea? How about a single wall with three long shelves - each say 18 inches above the other. The lowest level is a branch line terminus (from hidden sidings at one end) operated by tank locomotives and railcars. It has an adjoining ("walk across the tracks to") platform for another connecting line which just hapens to be the Fleischmann rack system. This takes passengers to another interchange on the top level with an HOe line. A kind of 'Lithgow Zig Zag' (my local narrow gauge railway) with interchanges instead of switchbacks. OK, it is not plausible or prototypical but I am starting to get carried away with this - the track planning materials are almost coming out. Let's see - lower line SBB electrified, rack loco the Fleischmann steeple cab; top level HOm Bemo, with transporter wagons thrown in somewhere!. I'll stop now in case I start taking myself seriously. Best wishes, Richard