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.