Just in case anyone else ever needs to go from Barcelona to Santander on bike friendly trains, here's a way I think will work. Though I expect there is a way of doing it in less than 3 days!
Day 1: Barcelona to Zaragoza: 8.47 to 14.23 on R Express
Day 2: Zaragoza to Logrono: 6.30 to 8.38 on R Express
Logrono to Burgos – 16.06 to 17.50 on a Talgo
Burgos to Palencia – 20.20 to 21.14 on R express
Day 3: Palencia to Santander – 10.26 to 13.34 on Regional
I think that just looks a bit too slow, is there any guarantee you'll even get the bike on all of those regional trains?
If not may'be look at tgv again.
To travel from the bottom of France to the top can be done in a day, barcelona up to the french border is something like 100+ miles.
As i said you can do the whole lot in a single day but it would be a long day with a very early start.
Die bahn is still the best website i can think of for european train travel.I Have done around 25 000 to 30 000 miles on european trains ,so had to do a lot of searching on die bahn.
barcelona to zaragoza is quicker than that i think, using the quick service, 2 hours or something.
I'll have to go through this type of thinking for my Gibraltar trip as i have no plans to cycle portsmouth-gibraltar and all the way back to portsmouth.
You should also be able to get the train right along the basque coast, i stayed in Biarritz, southern France, the spanish border isn't that far away [30 miles i think], a train goes to Irun from Biarritz, then other trains [including a train that looks like the ones on london underground] go along to san sebastian, which has amazing beaches, after that you have Bilbao where the guggenheim is.
Bilbao to santander [around 58 miles apart] must be possible, hugging the coastline.
The services in this area aren't all that fast mind you, it's still not a 3 day tour though.