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Renfe is the main rail operator but don’t forget to check the prices on the low-cost train routes as well (OUIGO, Iryo) on the main Seville-Barcelona route.
When I went in May, Iryo was new so they beat Renfe out on prices for any Madrid-Seville, and Madrid-Barcelona routes.
Ended up doing 1st class in Iryo pretty cheap which came with a meal that saved me hunting down food at my destination.
Also, the Renfe website is notoriously bad using it with US credit cards but just keep trying with different browsers/tabs and eventually, you’ll get something to go through.