Swedish sawmill and paper manufacturing companies operated a plethora of steam tugboats of various sizes to transport timber to the manufacturing sites up until app. 1950.
There never was a tugboat named Norrsundet but certainly a number of tugs operated from the Norrsundet harbor witch is the end terminus of DONJ.
The real counterpart of the model is the Steam tugboat Ströms Bruk witch was built in 1917 and had a working life up until, 1967. She was finally scrapped in 2009.
Se: http://www.tugboatlars.se/Stromsbruk.htm
DONJ was a 891 gauge railroad which operated between 1876 and 1970. It ran between Linghed in Dalarna and Norrsundet at the Baltic coast.