I think the most recent MTH conventional operated offerings were in the Lionel Corporation Tinplate (LCT) line that came in either proto-3 (xx-xxxx-01) or conventional build-a-loco (xx-xxxx-0) form. IIRC the last MTH non-tinplate/conventional - aside from handcars and bump-n-go trolleys - was during the first couple years of Protosound-2 (PS2) with their Locosound offering (also xx-xxxx-0) that had some PS2 tech providing the smoothness of cruise-control but with conventional e-unit and horrible steam/diesel engine sounds.
I strongly believe that MTH should have differentiated its product ID numbering system due to the confusion created by the multiple use of the x-1 and x-0 product ID suffix across MTH's evolving products including the various generations of TIU's.