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It's easy....ish. Met a few people who'd scratched TPR back in Girona this year, who'd had to get back from places like Jaca where there's no straightforward way of putting a bike on a train.
They'd been told there was no way they were getting on one bus, only for another driver to say if the bikes were wrapped there would be no problem. One guy had basically cling filmed and bin bagged his bike and was finally let on a Spanish bus.
Apparently it is easy to put bikes on buses in Spain and there is a direct one from Santander, so ferry + bus, rather than train, would be the best way there without flying.