Train travel between Bordeaux and Barcelona with stop at Carcassonne.

Can't speak for the train travel part, but we spent two nights in Carcassonne. We had a car and stayed at an airbnb about 10-15 min walk away from the fortress and about the same to the 'new' town.

The main reason we stayed two nights was because we weren't going to be arriving until after supper, so the first night was a wash. And it was early Oct, so dark by 7pm-ish. After leaving Carcassonne we were heading a few hours drive away so we didn't really want to be leaving after supper and thought it best to spend the extra night.

We spent about a half day exploring the fortress and a few hours in the 'new' town. Depending on your arrival/departure time and your interest in the fortress (nothing in the town part really excited us, others could beg to differ) one night would probably suffice. But obviously if you don't get in until the evening, you would probably want to stay until at least mid-afternoon the next day.

Then again, we did spend a few hours wandering around the shopping part in the area within the walls, so if you really only want to explore the fortress and skip the shops, I'd say 3-4 hours.

Not sure if you are into airbnb's, but I can send the info if you want it. It was more of an in-law apartment - separate bedroom/bathroom/small dining area, but attached to the main house (you had your own door to come and go). And close to the bus (again, we didn't use the bus, and the car was parked for the day - we walked everywhere)

It will be two tickets: Bordeaux-Carcassonne, and Carcassonne-Barcelona. The Bordeaux-Carcassonne is a 3-hour direct train. Prices run from 15-53 euro, depending on how early you buy the ticket. The second is a local train to Narbonne or Perpignan where you connect to the TGV to Barcelona; that's one ticket. Easy as pie. See voyages-sncf.com or Capitaine Train for info.

Several direct trains per day, but this one is best for you ...
IC 4657 Dep 09:28 BORDEAUX ST JEAN (France) Arr 12:31 CARCASSONNE (France)

Just the one direct train currently ...
RSC 9729 Dep 09:08 CARCASSONNE (France) Arr 11:31 BARCELONA SANTS (Spain)

I would stay 2 nights in Carcassone, perhaps here ... http://www.hotelpontvieux.com/#!accueileng/c1ber
Book your train journeys direct with sncf ... http://en.voyages-sncf.com/en/

Thanks very much for the travel info; very helpful.

It came quite coincidentally as I had looked up this very journey just yesterday.
I have a cousin now living in France between Lourdes and Biarritz, as well as very close friends in Barcelona.
With direct trains it came as a surprise how easily this journey can be made over 2 days.