Building one of these has been on a the list of things to do for a while, given a couple more decades it might happen. Currently it's blocked on building up a functioning workshop (lathes, mills, air, ...).
I've generally had a perchant for industrial narrow gauge steam, particularly slightly peculiar locomotives such as the various O&K ones, and Garratts. I like things to have practical use, they have to be able to carry a load and that means an assortment of passengers. It also has to be practical to move! Seven-and-a-quater-inch gauge is reasonably well deployed, at least in Europe, having compatibility means being able to use other people's tracks and to be able to track the locomotive "on the road" (or on the canal) and visit places.
Building standard gauge British locomotives at 1:8 gets a neat locomotive that still looks a bit petite, whereas building a 1524-millimetre Russian or Finnish locomotive gets something a tad bigger at 1:8 without ending up at "ride-inside" narrow-gauge proportions 1:4 proportions. ...and I've had a long interest in Finland, to the point of living there and intending to again.
Of Finnish locomotives, "Orion" is tempting, it's narrow gauge but spent some time on the Welshpool and Llanfair (an railway I have a slight, but passed affinity through its proximity to the 15-inch gauge railway I first played on.
Fuel-wise, I've seen the near instant-on (five minute) boil time for a propane/gas burner and the hour-long warm up for a coal burner. My own personal choice would probably be an oil-burner, the advantages of a quick start-up (although not as quick as gas) but with the safety of liquid fuel ...one must of course vapourise it to burn it.
Very tempting to build since it can be built modularly in three parts; first the power bogies, then the boiler complex later. In the mean-time it can be run off compressed air or an external boiler setup. When transporting, the weight is split across three sections and using high-pressure quick-connectors this shouldn't be a problem.
1910s/1920s shunting locomotive. There are still a few around of the second batch; the "Vr" stands for "heavy Shunter".
No.665 is in Running condition, No.663 is a beautiful 1:5-scale model at the Finnish railway museum. No.666 is Jan-Eric's 1:8 model which I've had the pleasure of driving on the museum track. There are a various other bricked full-size locos; at Kotka, Porvoo and a petrol-station to the west of Tampere. One even made it to Britain along with the other Finnish material at the Epping-Ongar railway.
Just like the Garratts, this is all-wheel drive. Rather than duplicating what Jan-Eric has, one option might be to add another a bit more weight, at the cost of carry-ability, and do the 0-6-2 Vr2 which was a stretch version of the Vr1. Aesethically the Vr2 doesn't have the characteristics of the Vr1, it's slightly over-length and the wood and water tanks are unattractive, along with the solid cab steps. All-in-all a loss of beauty and a loss of compactness for the sake of diversity, but it's a possibility.
In terms of engineering practicality the Vr1 and Vr2 locomotives both have double-ended/double-supported cyclinders, which provides a bit more support and ease in keeping everything centred.