I don't think there are duplex trains that go to Barcelona? I've never seen one in any case. So no upper level. And yes that would be an awfully long day to spend on the train. I would schedule a stop in Paris or Lyon to spend the night and visit a bit.

And no, it's not very scenic. There a bit of rolling hills as you go past Le Creusot in Burgundy and a bit of the Med as you get down around Perpignan. Other than that it's just flat farmland.

If you aren't going to visit France, then there's little point in taking the train. Better to fly.