Class 66 was put into service on the Østfold Line on 23 June 1946. It reduced travel time to Halden to 1 hour and 53 minutes, and was branded as the "Østfold Express". From 16 February to 8 September 1946, it also had three weekly services that continued to Gothenburg, Sweden, as the "Gothenburg Express". This was accomplished in 5 hours and 5 minutes. The high speed prohibited standing, and all passengers were required to purchase a seat reservation for . Three units were in operation at any given time, while the fourth was at service or in reserve. This proved to be too optimistic—during the 1950s, there were several periods were the trains were taken out of service due to lack of maintenance. During 1954, Saturday trains were operated as double units.