The story
The concept was defined in the Prussian Kleinbahn law of 28 July 1892, that was designed to encourage the construction of local railway lines by private companies. The word Kleinbahn was chosen by a majority of MPs in the Prussian parliament instead of a range of other options - Lokalbahn (local line), Bahn unterster Ordnung (line of the lowest order) or Bahn untergeordneter Bedeutung (line of secondary importance) - because it was neither a foreign word nor had negative overtones.
In several former German states such as (Mecklenburg, Oldenburg and Baden) the concept Kleinbahn was partially adopted for lines of limited length and light construction. In the other German-speaking states, the terms Lokalbahn (Baden, Bayern, Austria), Sekundärbahn (Saxony) or Vizinalbahn (Bayern) were preferred.
In everyday speech the term Kleinbahn is widely used as a synonym for narrow gauge lines or garden railways. Despite that, Kleinbahnen may be built in either 1,435 mm (4 ft 8+1⁄2 in) standard gauge as well as in narrow gauge.
From Wikipedia’s article on Kleinbahn, shared under CC BY-SA 4.0.3