The difficult geography of the region required the line to be built in sections. The first to be opened, on 10 March 1876, was between Bourg-en-Bresse and Simandre-sur-Suran, about 22 km long. On 6 July the same year, the line was extended 3 km to reach Bolozon. These 3 km include the two significant engineering works on the line, the Racouse tunnel (1.7 km) and the Cize-Bolozon viaduct (245m long) over the river Ain at a cost of 339,000 francs. This multi-arch viaduct is unusual in that it carries the railway line and, on a lower level, the road. Later on, three new tunnels extended the line from Bolozon to La Cluse, including the Mornay tunnel, the longest on the line at 2.6 km. The line from Bourg-en-Bresse to La Cluse was opened on 29 March 1877 by the DSE.