The driving force behind the project was the speculator Count Adrien de Lavalette (Fr).
The Ligne d’Italie opened its first section from Le Bouveret (at the southeastern end of Lake Geneva) to Martigny via Saint-Maurice on 14 July 1859.
The next section from Sion to Sierre opened after some delay on 15 October 1868, bringing the total length of the line to 79.5 km.
The line was extended from Sierre to Leuk on 1 June 1877 and to the provisional terminus in Brig on 1 July 1878.
In 1890, the SOS merged with the Jura-Simplon-Bahn (JS), which significantly accelerated the efforts to construct the Simplon Tunnel.