To get the essence of Norway, you need to combine train, bus and boat. The trains are all dead ends, due to the terrain (fjords and mountains). If you're up for it, and depending on season, walking and skiing really add to the experience (see the
Norske Turistforening), and can be the least expensive way to do it. Norwegian friends in Seattle turned me on to this, and I've done it: skied to the top of Norway in late winter, and walked across from west west to east in summer! A real cultural experience!
As a side note, and even the trolleys in Oslo, are equipped to carry lots of outdoor gear, as they are sports crazy there. When I first arrived in Oslo, there were huge racks of skis in the train station, all tagged with their owners' spring vacation destinations. They were sent on ahead, and were waiting in racks at the destination stations a week later when I went on my ski tour. I saw people looking through the racks for their skis, and never heard of anyone taking skis that did not belong to them.
It was fun to watch people walk up to the Oslo trolleys, slap their cross-country skis into the rack on the side, and carry their poles on the trolley car with them. I hope they still do that. Also, it's dark very early in winter, so they have lighted ski tracks all around town, and take the city trolleys up to ski after work.