I took a pass at the LEGO Digital Designer, and for my first and second efforts, designed a modular cargo system in a general space theme. The basic premise is that cargo containers link together, are driven by an autonomous command module, have a communications module in the front, and can be piloted by a tug for close maneuvering.
This first version of the system was based on a shipping container, to which a command module with one set of engines was attached, and a set of slave engines at the bottom, and the communications module at the front. Of course, the points where the engine modules attached would fit neatly into the holes where the communications module attached, so the train could be extended by any number of cars. The whole thing ended up boxy, and with multiple cars attached there was little indication where one car ended and the next began.
For the second version, I redesigned everything except the two components I was happy with: the tug, and the communications module. I elongated and angled the cargo containers so that they would be more distinct when they were joined, and I removed the slave engines and made a single command module. In this version, the end result looks more like a train, especially when many cars are added (a task which is needlessly difficult in LDD). Best of all, the components from this train fit with the components from the first version, so it is possible to mix and match.
This was the idea that started it all, a little tug that would be used by experienced pilots to maneuver clumsy or automated ships in tight orbits. Checkered like a taxi cab, for no real good reason. Pilot's neckerchief ditto.
Four tugs can dock here for maintenance, or just a place for their piltos to relax.