There currently is one option to travel by night train from Trondheim (Norway) to Oslo (Norway), taking between approximately 7 and 8 hours: Namely the 406 operated by SJ.
The Swedish Railway SJ operates night trains daily throughout Sweden, covering long distances and serving remote areas.
SJ night trains offer seats, couchettes, and sleeper cars. The sleeper cars feature various compartment types: for two or three people and single compartments. On some routes, Deluxe compartments with private bathroom and shower are also available.
Each sleeper compartment has a washbasin. The Deluxe option additionally provides a private bathroom with shower.
The couchettes offer space for up to six people with access to provided bedding and toilets in the carriage corridor. The middle berths fold up during the day to create seating areas. Compartment options include women-only, men-only, or mixed assignments. Private couchette compartments are available for travelers with pets upon request.