Geared steam locomotive
A geared steam locomotive is a type of steam locomotive which uses gearing, usually reduction gearing, in the drivetrain, as opposed to the common directly driven design.
Live steam
The generally accepted smallest gauge for a live steam locomotive is Gauge 1, although O scale live steam models are also common. 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.