So-called "small scale" live steam is actually on the large end of the usual model railroading spectrum, though most live steam equipment runs on much wider track (.75" scale or larger). Live
steamers are easy to find in gauge 1 (45mm or "G" gauge), less common but available in 0 gauge (32mm).
Gauge 1 live steamers are popular with garden railroaders because they interoperate with electric gauge 1 equipment and because some of the problems with live-steam operation are lessened outdoors.
(Most people don't have room for a .25- inch gauge track in the basement; spillage of hot water or oil drips are more problematic on indoor layout materials.) Some larger scale steam clubs also have
gauge 1 tracks for the smaller steamers. Because of the interoperability, most gauge 1 live steamers seem to belong to clubs with other gauge 1 modelers, rather than specialized gauge 1 steam clubs.
(The folks who run electric models really seem to like the live steamers, too!)
What makes up a gauge 1 live steamer? They share the basic characteristics with all live steamers: they burn some sort of fuel to boil water and move cylinders. They may differ from gauge 1 electric
trains in that they typically have working parts such as valve gear, pressure gauges, relief valves, feedwater pumps, etc. They may differ from larger live steamers in that their mechanical design is
(often) simpler. Fuels are usually methyl alcohol ("meths"), butane or solid fuels (rare). Modeling detail spans a wide range: a live steamer will run with no more than a boiler, cylinders, valves
and pushrods. Aster and others make super-detailed scale models. There are many other models which fall in between these extremes. Gauge 1 live steamers typically pull rolling stock which is also
seen behind large-scale electrics: LGB, Bachmann, scratch-built.
There are many technical distinctions among live steamers which will affect both price and performance. Some examples: boiler types may be "flue", "pot-boiler", "porcupine", "Smithies", etc. Each of
these represents a trade-off of construction complexity (and thus cost) vs. efficiency. Some locos have two steam cylinders, some only one. Those with one sometimes have dummy cylinders where they
would be on a prototype, with the working cylinder between the frames. Cylinders may be fixed (with a wristpin on the driving rods) or oscillating (the driving rod is a single piece, and the cylinder
moves to accomodate the movement of the attachment point on the driving wheel).
Because of the relative complexity of steam locomotives, it's usually a good idea to chat with someone who knows about them, or to do some learning about them, before buying one. Below are some
resources which can help.
On the horizon: gas-electric locomotives. The "large scale" live steam community (1:12 or 1:8 scale) have been running gas-electric or diesel-electric locomotives for some years. These are now
starting to appear in the smaller scales, too. These locos model modern diesels in their operation: an internal combustion engine provides power to drive an electric generator in the locomotive cab,
which in turn powers the traction motors which drive the locomotive's wheels. Kosaku Wada of the Yokohama Live Steamers club (Japan) has developed a gauge 1 gas-electric using an engine developed for
radio controlled helicopters. The prototype is converted from an LGB European box-cab diesel model, and is a smooth runner and a strong puller. Mr. Wada has arranged for representation in the USA,
for those interested in going to completely self-contained (and prototypical) power sources. (No products have been announced yet.)