The Thumer Netz was a narrow gauge railway network in the area around Thum in Saxony, Germany that operated from 1886 until 1975. It had a 750 mm (2 ft 5+1⁄2 in) gauge. Total length was about 44 km (27.34 mi).

This network had three segments that connected three standard gauge stations: In Wilischthal and Schönfeld-Wiesa were connections to the Annaberg-Buchholz-Flöha railway, and in Meinersdorf was a connection to the Chemnitz–Adorf railway. The three segments had their hub in Thum. 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.

A landmark was the 180 m (590.6 ft) long and 35 m (114.8 ft) tall Greifenbach viaduct between Ehrenfriedersdorf and Geyer.

This was the first segment to be finished for the Thumer Netz. Construction started in September 1885, and operations started on December 15, 1886. Length of this segment was 13.54 km (8.41 mi). The track started at the Wilischthal station in Zschopau on the Annaberg-Buchholz–Flöha railway, located in the Zschopau valley, and wound its way along the Wilisch river through Wilischau, Grießbach, Venusberg and Herold into Thum.

This segment operated until 1972. The tracks between the paper mill in Wilischthal and Thum were slowly demolished in the following years. The short remaining section operated until 1992 as an industrial siding, connecting the paper mill with the regular gauge tracks in Zschopau. This short section of tracks still exists today, but is in unusable condition.

This segment was opened in 1888 and featured the Greifenbachtal bridge, at the time one of the largest narrow gauge bridges in Germany. The initial segment started in Schönfeld-Wiesa and ran via Tannenberg to Geyer, parallel to the roadway between the towns. In Geyer the tracks made a wide 180 degree turn through the middle of town to arrive at the station in Geyer. In 1906 the track was extended to Ehrenfriedersdorf and Thum, which obsoleted the short connection between Herold and Ehrenfriedersdorf on the Wilischthal-Thum segment. In Thum, a new station was built to connect the new railway to the already existing segment.

Passenger service operated until 1968. Sections were dismantled over the following years. After 1972, only the short distance from Schönfeld-Wiesa to the paper mill was still being used as an industrial siding. In 1985, this short section was upgraded to regular gauge tracks.

This segment of the Thumer Netz was finished in 1911 and connected the existing two segments with the Chemnitz–Adorf railway. 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. Total track length was 12.5 km (approx. 8 miles). By the early 1970s the tracks were in relative disrepair and had many slow zones. Last passenger operation was in 1974 and the track was dismantled in 1976, which marked the end of the Thumer Netz.