There's something very special about walking into a train station in the heart of a European city, and seeing a departure board that looks like this: http://www.enjoy-europe.com/hte/chap17/images/P1020015-Bruss...; the sense that you could just buy a ticket and jump on a train to anywhere. You tend not to get the same feeling from an airport departure board, where for the most part you couldn't just buy a ticket and fly to any of those destinations.
I'll second Oslo to Bergen. We did it in reverse, as part of the 'Norway in a Nutshell' itinerary which is about 12 hours (can be done over two days) because it replaces some of the rail journey with a fjord cruise, the Flam railway, and a mountain coach.
In Europe, the convenience/inconvenience of airport locations varies a ton (which does influence purchase choices somewhat). For example Frankfurt's airport is only 8 minutes by commuter rail from the city center, and Copenhagen's is 12 minutes. But Paris-CDG and London-LHR are far from their city centers.
You still have to trek through the airport, wait in security lines etc etc. so in reality it takes much longer than the time it takes the train to travel from the airport to the city. That said flying is often if not nearly always cheaper for short hauls in Europe.
This is very relaxing to have on in the background.