After almost 40 years on the throne, he will be succeeded by his son Prince Felipe.
The magistrate formally charged Garzon with 79 counts of homicide and numerous offences of bodily harm.
A judge in Santiago de Compostela was assigned to investigate the accident.
Bodies covered in blankets lay next to the overturned carriages as smoke billowed from the wreckage.
Spaniards marched to protest deep austerity, corruption & the privatisation of public services.
Spain's opposition Socialists have told Prime Minister Mariano Rajoy to resign.