A 7 1/4 inch gauge GWR 0-4-0 built to Gentry's "Midge" design, but in wood and tin. What you lose on driveability you gain on never needing a boiler test (although it could use a splash of creosote round the axle boxes now and again when you do your fences).
Pictures of a real one in the archive here.
gauge | 7 1/4 inch |