There currently is one option to travel by night train from Stockholm (Sweden) to Narvik (Norway), taking between approximately 18 and 19 hours: Namely the 94 operated by SJ.
The Swedish Railway SJ operates night trains daily throughout Sweden, covering long distances and serving remote areas.
The night trains include seating cars, couchettes, and sleepers. The sleeper cars provide compartments for two or three people as well as single compartments. Some routes also offer Deluxe compartments with private bathroom facilities and shower.
Each sleeper compartment has a washbasin. The Deluxe option additionally provides a private bathroom with shower.
Couchette compartments accommodate up to six people. Bedding is provided including pillow, pillowcase, sheets and blanket. During the day, middle berths are folded up so lower berths can be used as sofas. You can choose between women-only, men-only, or mixed compartments. SJ offers limited couchette compartments where pets are allowed, and these must be booked as private compartments.