The driver -- whose name was reported Friday as Francisco Jose Garzon Amo -- was charged with 78 counts of reckless homicide. He has been in police detention at a hospital since Thursday after the eight-car train derailed near Santiago de Compostela.
Interior Minister Jorge Fernandez said there were "rational indications" the driver was at fault, CNN reported. Fernandez declined to comment on what those indications were.
Spanish media have quoted investigators as saying the train was going 120 mph at the time of the derailment.
The speed limit on the bend of the track where the train derailed was 50 mph.
A U.S. citizen, Ana-Maria Cordoba, of Arlington, Va., was among the dead, while at least six U.S. citizens were injured.
U.S. President Barack Obama Friday expressed his gratitude to the Spanish government for aiding the injured Americans, ThinkSpain reported.
"Today, the people of the United States are crying along with our Spanish friends, who are in our thoughts and prayers," Obama said in a letter to Spanish President Mariano Rajoy. "We are willing to offer any assistance that we can in the difficult days ahead."