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.
Steam motor
Other beneficial characteristics were commonly found, but were not essential.
This was usually poppet valves driven by camshafts. In the steam motors built by Sentinel, the motor was derived from their already advanced steam wagon design.
In a conventional steam locomotive, the 'gear ratio' is set by the size of the driving wheels. As a steam motor uses a geared drive, the wheel size can be reduced. This makes for a lighter and more compact chassis, particularly by reducing the unsprung weight of large wheels. Small wheels also allow the motor to be mounted on a bogie within a passenger coach to form a railcar, rather than the large wheels being the size of a locomotive.