Perhaps of curious interest... There is a guy on Ebay that re-gauges some old Athearn HO diesels to HOn3. The look neat - but HUGE compared to HOn3 rolling stock, and probably would require modifying the ROW on one's layout. I'm told the conversion isn't that difficult for the older Athearns that had the outside bearings; the newer Athearns won't convert the same way. Even though it is a model, it's an analogy for some of the challenges a full-size railroad would face - some trucks can be modified, others cant. Also - the end result may be simply too big for the ROW.
Kevin