The majority of the railway network was single track and its equipment and rolling stock was mostly in poor condition and unable to cope with increasing traffic. The difficult financial situation during the first half of the 20th century limited the modernisation of the Swiss rail network. The main work carried out was electrification, duplication and safety improvements. Electrification started on an experimental basis in 1888 and was completed in 1960. It was accelerated as a result of coal shortages during the two world wars. Of particular note was the early electrification of the Bern-Lötschberg-Simplon line, which was opened in 1913 with single-phase operation. At the beginning of World War II, 77% of the Swiss rail network had been electrified, while other European railways had a level of electrification of 5%.
The construction of tunnels shortened distances and improved gradients, allowing the improved handling of traffic growth. The Simplon tunnel between Brig and Iselle, Italy, was opened in 1906. The Mont-d'Or tunnel was opened between Vallorbe and Frasne, France, in 1915 and formed part of the route of the Orient Express between Paris and Istanbul from 1919 to 1962. The Hauenstein base tunnel was opened between Olten and Basel in 1916.