Regiontog are long distance express trains operating on the main lines throughout Norway. There are different kind of trains used for these services, basically you will find two different variations: long-distance services from Oslo to Trondheim, Bergen and Stavanger as well as from Trondheim to Bodo are operated with either recently refurbished carriages or EMU. They offer seats in 2nd class as well as special "NSB Komfort" seating, comparable to 1st class with wider seat pitch and free drinks and newspapers. All trains have a bistro, plugs and wifi, as well as special compartments for the disabled, children/families and pets. On shorter distance services other trainsets are used which usually only offer 2nd class and "Komfort" seating, plugs and wifi but no bistro car (but vending machines). ユーレイル
The price is the same for all distances. If you travel with reservations, you will need one per connection. Example: a connection with change of train (ie Trondheim - Bergen via Oslo) will need just one reservation.
There is no compulsory reservation on Regiontog trains, however a reservation is strongly recommended especially for longer distances. If you want to travel without a reservation you can ask the conductor if and where there are free seats.
Oslo - Bergen/Stavanger/Trondheim/Halden | Trondheim - Bodo | Lillehammer - Oslo - Skien | Dombas - Andalsnes | Oslo - Göteborg