The Thumer Netz was a narrow gauge railway network in the area around Thum in Saxony, Germany that operated from 1886 until 1975.

This set of railroads brought much-needed connectivity and transport capacity to the small towns and villages of the central Ore Mountains, which had formed a viable industry in the 19th century due to abundance of water power, wood, ore and other natural resources as well as cheap labour.

The short remaining section operated until 1992 as an industrial siding, connecting the paper mill with the regular gauge tracks in Zschopau.

The initial segment started in Schönfeld-Wiesa and ran via Tannenberg to Geyer, parallel to the roadway between the towns.

Starting from Thum, the track went uphill via the neighboring Jahnsbach into Hormersdorf and from there downhill again via Auerbach and Gornsdorf into the Meinersdorf station.

When the first narrow gauge locomotives were delivered to the Royal Saxon State Railways by the Sächsische Maschinenfabrik in Chemnitz, there was no official type system yet.

The letter K (for German klein - small) was appended to existing types to indicate them as narrow gauge locomotives.

Some more locomotives of this type were used between Schonfeld-Wiesa and Geyer, but due to the lack of power they were soon replaced by the Saxon III K. This type had sufficient power for the windy and step tracks, but because of their complicated design, starting in 1892 they were replaced by the Saxon IV K, which went on to become the most numerous narrow gauge locomotive in Germany.

The first transporter wagons started service around 1912, and by 1945 completely replaced any of the narrow gauge freight cars.

It was one of the largest narrow gauge train bridges in Germany and crossed the Greifenbach valley approximately 1.5 km (0.93 mi) north east of Geyer.