Sweden is so bloody long. I've gone to Norway and Denmark by train. Denmark by train was roughly the same time as flying, including transfer etc. Too far for any other country really
The trick is to catch a sleeper train. Have a full day of work/leisure, board the train, sleep, and wake up at your destination in the morning.
The Stockholm-Copenhagen journey is short enough that they park the train somewhere in the middle of southern Sweden for an hour or two to make the timing more convenient.