The Gjøvik Line (Norwegian: Gjøvikbanen) is a branch line from Oslo going north to the town of Gjøvik. While there isn't a lot of train traffic on this line, it does provide a secondary route to Norway's second largest city of Bergen, so freight trains run on it and there's occasionally passenger trains detoured on the Gjøvik Line heading between Oslo and Bergen. The usual traffic on the Gjøvik Line is Vy's local and regional passenger trains operated with Stadler FLIRTs. While the variety of trains isn't a lot, the Gjøvik Line has amazing scenery shortly after leaving Oslo. The route passes through Nordmarka, a large area with dense forest and many hiking trails that can be accessed from stations on the Gjøvik Line.
This video shows the usual Gjøvik Line traffic of Vy's Stadler FLIRTs, but also a freight train, around the village of Movatn. It concludes with a scene at Kjelsås Station, which is typically where northbound and southbound trains meet.
The Gjøvik Line is my favorite rail line in Norway. Although the traffic isn't diverse, its winding route through nature and villages makes it great to explore both on foot and while riding on the trains.