Someone has mentioned "doodlebugs", AKA gas electrics. There were a wide variety
of these critters, from model T school buses on flanged wheels to Unit-Stanley, Alco,
and Baldwin steam coaches (these last of which I would love to see modeled). The
now delayed or abandoned MTH McKeen car was just one of these interesting
machines that once rode the rails. HO has always had a number of these available,
in brass or kit form, so it has long annoyed me that O three rail, with its large population of people with money to spend have never had much of a choice. I got into HO in my preteens, in the 1950's, so doodlebugs have not been an unknown and have always been of interest to me. Besides the MTH version offered a few years ago, in O scale there can be found the Walthers C&NW kit, and the Amercian Standard
Car Co. Brill (I think) kit. These two kits can be powered with three rail trucks, and Walthers made a three rail power truck that can be found with the kit. The ASC Brill, if it is a Brill, ran on a lot of railroads as did many other makes. AB and AC Mack railbusses were offered in several brass versions in Ho...none for three railers.
While I have little interest in diesels, the one odd ball I might buy is the Rock Island
cab-windowed B unit that broke off the Colorado Springs section from Chicago while the A and other standard units took the Denver section on from Limon. Another odd ball diesel was that CNJ? unit that looked like two A units butted up back to back with a cab on each end. I, in those early teen years, kitbashed a Marx #21 pair, with tinsnips, into a crude model of that. I think those double-cabbed diesels are common in Europe and Australia.
Other oddballs not found in "tinplate" are drover, combine, and side door cabooses.
There was a TREMENDOUS variety of these on assorted and obscure branch lines around the country, as well as on Class 1 railroads, such as the Missouri Pacific which had a great many different types. The Santa Fe (one of theirs sits by the Rhyolite ghost town station in Death Valley National Park) and UP also used them widely. Bobby Hall in Texas inported a few brass ones in O scale , T&P and Santa Fe, that can be three railed and I have scratch built several, models and freelance. Bob Peare Train Craft had a side door caboose in an O scale kit that shows up on eBay. Mine is on Lionel arch bar trucks.
And there, are inspection engines, those weird combinations of a steam boiler
front end and cab joined to a passenger coach. The one I'd love to model is on a photo over your head in the station in Shelburn Historical Park in Vermont. That one
looks like it might have run, like a gas electric, as a revenue earning critter.
I guess you could say oddballs are my favorite railroad rolling stock.