The Western Switzerland Railways (Chemins de fer de la Suisse Occidentale, shortened to Suisse-Occidentale; SO or S-O), were initially a joint operation of three Swiss railway companies, but these companies merged on 1 January 1872.
The SOS merged with the Bernese Jura Railways (Chemins de fer du Jura bernois; JBL) to form the Jura–Simplon Railways (Compagnie des Chemins de Fer Jura–Simplon; JS) on 1 January 1890.
This harsh competitive situation was compounded by financial difficulties due to construction cost overruns.
The financial situation of the three western Swiss railways stabilised and from 1868 onwards the association was able to pay a very modest dividend.
These lines were originally intended to form the western end of the Swiss National Railway (Schweizerischen Nationalbahn; SNB).
The funding was provided by the Societe Suisse pour l'industrie des chemins de fer ("Swiss Company for the Railway Industry"), which forced the SO to reorganise the administration.
The SOS, with investment capital of CHF 248 million and a network length of 581 kilometres, was the largest railway company in Switzerland at the time.
Its strategically most important goal was the building of a connection from Brig to Domodossola by tunnelling under the Simplon Pass, but this did not proceed for the time being.
On 21 January 1888, large masses of rock disintegrated at Cheyres and fell on the track creating piles of rubble two or three metres high.
The following map shows the Western Switzerland–Simplon route network before the merger with the Jura–Bern–Lucerne (JBL): On 1 January 1890, the SOS merged with the Jura-Bern-Lucerne Railway (JBL), including the Gümligen–Lucerne line, which belonged to the canton of Bern, to form the newly established Jura–Simplon Railways (JS).
The Swiss Confederation also participated in the new railway company by means of a voluntary share purchase.