Live steam is steam under pressure, obtained by heating water in a boiler. The steam may be used to operate stationary or moving equipment.
A live steam machine or device is one powered by steam, but the term is usually reserved for those that are replicas, scale models, toys, or otherwise used for heritage, museum, entertainment, or recreational purposes, to distinguish them from similar devices powered by electricity, internal combustion, or some other more convenient method but designed to look as if they are steam-powered. Revenue-earning steam-powered machines such as mainline and narrow gauge steam locomotives, full-sized steamships, and the worldwide electric power-generating industry steam turbines are not normally referred to as "live steam".
Steamrollers and traction engines are popular, in 1:4 or 1:3 scale, as are model stationary steam engines, ranging from pocket-size to 1:2 scale.
Railroads or railways
Ridable, large-scale live steam railroading on a backyard railroad is a popular aspect of the live steam hobby, but it is time-consuming to build a locomotive from scratch and it can be costly to purchase one already built. Garden railways, in smaller scales (that cannot pull a "live" person nor be ridden on), offer the benefits of real steam engines (and at lower cost and in less space), but do not provide the same experience as operating one's own locomotive in the larger scales and riding on (or behind) it, while doing so.
One of the most famous live steam railroads was Walt Disney's Carolwood Pacific Railroad around his California home; it later inspired Walt Disney to surround his planned Disneyland amusement park with a working, narrow gauge railroad.
The live steam hobby is especially popular in the UK, US, New Zealand, Australia, and Japan. All over the world, there are hundreds of clubs and associations as well as many thousands of private backyard railroads. The world's largest live steam layout, with over of trackage is Train Mountain Railroad in Chiloquin, Oregon. Other notable layouts are operated by the Los Angeles Live Steamers Railroad Museum and the Riverside Live Steamers.
Scale
A live steam locomotive is often an exact, hand-crafted scale model. Live steam railroad scales are generally referred to by the number of inches of scale per foot. For example, a 1:8 scale locomotive will often be referred to as a 1½" scale locomotive. Common modelling scales are Gauge 1 (1:32 scale), 1/2" (1:24 scale), 3/4" (1:16), 1" (1:12), 1½" (1:8), 2½" (~1:5) and 3" (1:4).
Track gauge
Track gauge refers to the distance between the rails. The ridable track gauges range from to , the most popular being , , , and (see Rail transport modelling scales). Some live steam club layouts use dual or even triple gauge tracks. Gauges from and up are called "Miniature Railways" (in the US these are known as "Grand Scale Railroads"), and are used mostly in amusement park rides and commercial settings.
Often the gauge has little to do with the scale of a locomotive since larger equipment can be built in a narrow gauge railway configuration. For instance, scales of 1.5, 1.6, 2.5, and 3 inches per foot (corresponding to scales of 1:8 to 1:4) have been used on a track gauge.
The generally accepted smallest gauge for a live steam locomotive is O scale. Producing smaller-scale models remains problematic, as the laws of physics do not themselves scale: creating a small-scale boiler that produces useful quantities of steam requires careful engineering. Hornby Railways has produced commercial live steam-powered locomotives in OO scale by utilising an electrically heated boiler mounted in the tender, with cylinders in the locomotive, and control provided by electrical signals fed through the track from a remote control unit. They are less mechanically realistic than models in larger scales; the visible valve gear is a dummy, as on the electric-motor-powered models, and steam admission to the cylinders is controlled by a rotary-valve servo inside the boiler casing, which is also a dummy. Nevertheless, the locomotive is driven by steam that is created on board the locomotive and is hence a genuine steam locomotive.