We flew in to Barcelona for a 4-day sight seeing trip prior to our Basque journey last August/September. Be sure to allow a full day in Monserrat (train from Barcelona was easy). If you love architecture and the performing arts don't miss seeing a performance at the Palau de la musica Catalana. Gorgeous!
One day before the start of our RS tour we took a high speed train from Barcelona ( left about 9:00 a.m.) with a stop in Narbonne (lovely lunch in this quiet town) then boarded for Toulouse and on to Bayonne. Comortable trains, easy trip. Close connections, but hey it is Europe and they run on time (or not all! Yes, we avoided strikes). We did purchase our tickets in advance (just single ride one-way). We did upgrade the first leg to first class. Cost for the whole trip was around 130 Eu = probably didn't need 1st class.
Also, there ia an inexpensive and quick train to St. Jean du Luz. We stayed a night in this village prior to the trip as well. If you can squeeze in the extra days it is worth it.
One of our best advetures. The guides were awesome!