It runs from Olten along the foot of the southern Jura range through Solothurn, Grenchen, Biel/Bienne, Neuchâtel, Yverdon-les-Bains and Morges to Lausanne.

The other is the Midland line (German: Mittellandlinie) which connects Olten via Langenthal, Burgdorf, Bern, Fribourg, Lausanne to Morges.

The sole remaining single section, south of Twann station and north of Neuveville, is the location of the new Ligerz Tunnel.

The oldest part are the sections opened by the West Switzerland Company (French: Compagnie de l'Ouest-Suisse, OS) in May 1855 from Yverdon-les-Bains to Bussigny-près-Lausanne and in July 1855 from Bussigny to Morges via Renens.

[2][3] On 7 November 1859 the Franco-Swiss Company (French: Compagnie Franco-Suisse, LFB) opened an extension from Vaumarcus to the village of Frienisberg, near Le Landeron on Lake Biel.