The Østfold Line () is a railway line which runs from Oslo through the western parts of Follo and Østfold to Kornsjø in Norway. It continues through Sweden as the Norway/Vänern Line. The northern half is double track and the entire line is electrified. It serves a combination of commuter, regional and freight trains and is the main rail corridor in the south of Norway. The Follo Line runs parallel to the Østfold Line in tunnel. The Eastern Østfold Line branches off at Ski Station and runs before rejoining at Sarpsborg Station. The line opened as the Smaalenene Line () on 2 January 1879. Stations were designed by Peter Andreas Blix. It was the first railway in Norway to predominantly build bridges and viaducts with iron. The line underwent upgrades from 1910 through 1940 in which the section from Oslo to Ski received double track, the permitted weight and speeds were increased and the line was electrified. From 1989 to 1996 the section from Ski to Sandbukta received double track and the maximum permitted speed increased to . Work since 2015 been under way to upgrade most of the line to high-speed, with the new tracks at least as far as Halden by 2030. Route The Østfold Line runs from Oslo Central Station through the counties of Oslo, Akershus and Østfold to the Norway–Sweden border at Kornsjø, covering a distance of . The line generally follows the west shore of the Oslofjord until Halden, where it passes through major towns of Ski, Ås, Vestby, Moss, Fredrikstad, Sarpsborg. The line is standard gauge and electrified at . It is double track from Oslo to Sandbukta north of Moss, a distance of , as well as past Rygge Station. There are three railway stations and three freight terminals along the line. At Kornsjø the line continues through Sweden as the Norway/Vänern Line. The term Østfold Line is most commonly used to describe the section from Oslo via Moss to Kornsjø. This is sometimes also referred to as the Western Line. Both uses exclude the section from Ski via Askim to Sarpsborg, known as the Eastern Line. At other times Østfold Line is used to refer to the entire network, both via Moss and Askim. Sometimes Western Line is used to only describe the section from Ski via Moss to Sarpsborg. Although the eastern and western branches were initially planned as equals, the