On 2 January 1879 the Østfold Line opened as the Smaalenene Line to Halden after it was approved in Parliament six years earlier. The line was extended to Kornsjø and connected to the Swedish Dalsland Line six months later. Then it became possible to travel all the way to Gothenburg from Oslo. The Eastern Østfold Line opened on 24 November 1882, which branches off at Ski and rejoins at Sarpsborg.
The Østfold Line is a part of the so-called Intercity Triangle, the area where the majority of the passenger traffic is operating within. The line needs increased capacity today and a new double track is under construction between Oslo and Ski, also known as the Follo Line. This line will be opened in 2022 and shorten the travel time with 11 minutes.
Sources