It serves a combination of commuter, regional and freight trains and is the main rail corridor in the south of Norway.

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.

[3] The Eastern Østfold Line is 78.9 kilometers (49.0 mi) and runs from Ski to Sarpsborg via the municipalities of Tomter, Hobøl, Eidsberg, Mysen and Rakkestad.

[4] Østfold Line turns southward right after Oslo Central Station and runs alongside Bunnefjorden`s shoreline until Ljan, where the route takes a more eastern inland corridor to Moss.

From there on, heading northeast, the line uses the Glomma valley to Sarpsborg, bridges the river, turns southward and reaches Halden after running through mostly flat terrain in southern Viken (county).

The lack of early interest in a railway in Østfold was caused by the ice-free Oslofjord and the perceived non-necessity of build a line where a suitable waterway already existed.

[5] Preliminary surveys were carried out the following year, which also investigated two routes to the Swedish border, one via Tistedalen and one along Iddefjord.

Traders and politicians there were more concerned with being connected with the Swedish province of Dalsland, from which a large part of the town's lumber export originated.

To secure the Swedish part of the link, the Dalsland Line was built as a private railway with eighty percent Norwegian capital and its head office in Halden.

[19] A commuter train service was introduced from Oslo to Ljan Station from 1883, allowing the areas of Nordstrand to open up to large-scale housing.

[20] The railway allowed for faster postal services and the trains carried markers with the weather forecast, which was announced at the stations.

[24] To avail the problem in Oslo with human manure, depots were built at Drømtorp, Ås and Vestby to give farmers access to the resource.

[27] The higher demands caused the railway to order nine new NSB Class 27 locomotives, delivered between 1910 and 1916, which allowed the maximum speed on the line to increase from 70 to 75 km/h (43 to 47 mph).

[28] NSB introduced Ea 1, an accumulator electric locomotive, on local services between Fredrikstand and Skjeberg in 1916, where it remained in use until 1920.

[29] As the main international railway out of Norway, the Østfold Line had some of the highest demands for speed and axle loads.

In an effort to increase train weights and speeds, the Norwegian State Railways approved an upgrade plan in 1910 which was initially to be completed by 1919.

[30] The break-out of the First World War led to material and funding shortages,[31] and the upgrades were not completed until a new Sarp Bridge was finished in 1930.

NSB therefore decided that the Østfold Line, and especially the section closest to Oslo, needed to receive increased capacity, electric traction and more stations.

[38] Electrification was carried out in several smaller steps, with the first part from Oslo Ø to Ljan completed on 9 December 1936 and the last section from Sarpsborg to Halden, on 11 November 1940.

[33] NSB Class 66 was introduced as an express service on the Østfold Line, running a round trip from Oslo to Halden each day, bringing travel time down to two hours.

[40] The international trains were from 1948 served with the Swedish State Railways' SJ X5 units, able to run from Oslo to Copenhagen in less than ten hours.

[43] A regular train with pyrite started running in 1966, hauling up to 600,000 tonnes per year from Hjerkinn on the Dovre Line to Borregaard.

This involved limiting stops to Rygge, Råde, Fredrikstad, Sarpsborg and Halden south of Moss, causing a large number of stations to be closed.

However, this would not be sufficient to meet future needs, and in 1985 Parliament passed the construction of a double track from Tveter via Vestby to Kjenn.

[48] These plans were initially proposed as merely doubling the track to increase capacity, but NSB soon started looking at also raising the speed and standard.

[53] Peter Andreas Blix was hired as the national railway architect in 1873 and awarded the task of designing the stations on the Østfold Line.

The line from Haug to Seut north of Fredrikstad is planned to be finished within 2029, two years before the double track to Sarpsborg.

[55] The National Rail Administration is working on possibilities of expanding the double track further south and in conjunction with Swedish authorities complete a high-speed link between Oslo and Gothenburg.

[57] The Østfold Line has the largest potential for an increase in freight traffic, with an estimated eleven trains in each direction per day.

[58] Two steep hills, Brynsbakken on the Loenga–Alnabru Line and Tistedalsbakken southeast of Halden are currently major obstacles, which limit train size and set locomotive requirements.