Book train travel from Barcelona to Marseilles at the Rail Europe Web site (see Resources below) or call (877) 257-2887.
Review the train schedules and fares for Talgo trains from Barcelona Sants train station to Marseille St. Charles station (Gare de Marseille Saint-Charles).
Review the amenities available and choose your class of travel.
Print out the confirmation page and bring it with you to the train station.
Be aware that you'll change trains at the Montpellier St. Roch station. The entire trip should take between 6 and 7 hours.