The main line of the narrow gauge network runs from Palézieux on the SBB main line from Bern to Lausanne via Châtel-St. Denis and Bulle to Montbovon. The first section from Palézieux to Chatel-St. Denis was opened on 29 April 1901 to be followed by the section to Montbovon in 1904. The main line, which has a length of , makes connection with the Monteux Oberland Bernoise Railway (MOB) at Montbovon. The branch from Bulle to Broc was opened on 24 June 1912 to serve not only the village but the Cailler (now Nestlé) chocolate factory. The gradients in the line are not severe with a maximum of 1 in 31 on the main line; however, the section between Broc village and Broc Fabrique has a gradient of 1 in 20 descent to the valley floor to reach the chocolate factory. These lines, operated by electric traction from the beginning, were run by the Chemins de fer électriques de la Gruyère, CEG, a company which, in 1942, joined with the two standard-gauge lines to form the GFM. Originally the operating voltage was 750 V DC but this was increased to 900 V DC to give better hauling capacity on the line.