Francisco Jose Garzon Amo, who was hospitalized after the derailment, was detained by police in the hospital Thursday on suspicion of reckless homicide, the BBC reported.
Investigators say Garzon, 52, was going at least twice the speed limit around a bend, causing the train to derail near the Spanish city of Santiago de Compostela on Wednesday, killing 78 people and injuring 140 others.
Garzon faces 78 counts of reckless homicide.
Garzon will appear in court to be questioned by a judge, who will decide whether to remand the driver as an official suspect, release him on bail, or free him without charge.
Witnesses to the derailment say Garzon apologized profusely after the accident, ThinkSpain reported Sunday.
Garzon was heard saying "I'm sorry," "I've messed up," and that he "wanted to die" at the scene of the accident.
During a telephone conversation, Garzon was heard saying, "I've derailed, what am I supposed to do about it, what do I do."