an annual production of of iron ore. This was transported by truck to Treungen, where it was loaded onto trains. During the Second World War, non-local travel was not permitted without a permit. One of the large attractions was weekend trips to pick berries; at the most 1,000 people took the train from Arendal with this in mind. The cost to change to standard gauge was estimated to NOK 1.3 million; this was deemed too expensive by the ministry in 1938. NSB bought 20 boxes for lumber and 16 boxcars with modules which could be easily transshipped between two cars. The decision to convert the gauge was made in 1942 and some of the necessary funds were granted. The change of gauge started on 8 July 1946, which also including increasing the axle load to . Trains ran to Simonstad, then Åmli and Treungen, as the conversion was gradually completed. The first through train ran on 20 July. From 1950, NSB introduced an express train twice per day between Oslo and Kristiansand, using Class 66 units. A corresponding service was put up between Arendal and Nelaug; it ran without intermediate stops and used 38 minutes. In 1951, the line received two, and later three, Class 87 multiple units. The milk routes were terminated in 1952. In 1953, a new road from Fyresdal to Tjønnefoss opened, giving increased traffic from the station. In 1955, the last of the Cmb 13 trains were retired and a new Class 86 train was stationed in Arendal. It ran on the lower part of the line. If it was at service, a steam locomotives—normally Class 21—and carriages were used, which would cause delays. On 17 June 1957, the tunnel between the town square and the station in Arendal opened. From 1950 to 1957, NSB replaced the tracks south of Nelaug with heavier 38 kilograms per meter (25 lb/ft) tracks. Heavier Class 18c locomotives were introduced, but these could only be used south of Nelaug. In 1960, the upper part of the line received a Class 86, and in 1965, a common reserve Class 86 was stationed in Drammen for Stavanger, Kristiansand and Drammen Districts. The Grimstad Line was closed from 1 September 1961. As part of NSB's campaign to retire steam locomotives, a Di 2 diesel locomotive was test-run on 16 February 1961. It ran on the Arendal–Nelaug section for a month in 1962, was