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.
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From Olten the midland line connected to Bern and Thun, the Olten–Brugg line connected with the Swiss Northeastern Railway's (German: Schweizerische Nordostbahn, NOB) line to Zürich, the Jura foot line connected to Biel and the Olten–Lucerne railway line connected to Lucerne.
In May 1855 it opened the line from Bussigny-près-Lausanne to Yverdon and on 1 July 1855 from Bussigny to Morges via Renens as part of the Jura foot line.