Francisco Jose Garzon Amo, 52, who has remained in hospital under police supervision since the crash, was formally detained by police on Thursday night accused of criminal recklessness, according to the head of the Galician police service Jaime Iglesias.
The train was said to have been travelling at more than twice the 80 kilometres per hour speed limit when it came off the rails four kilometres from the city's main station and slammed into a concrete wall on the side of the track.
Amo, while still trapped in his cab, reportedly told railway officials by radio that the train had been travelling at 190 kilometres (118 miles) per hour.
"I was going at 190! I hope no one died because it will weigh on my conscience," he was quoted as saying by Spanish newspaper El Pais.
On top of the 78 confirmed deaths, 81 people remain in hospital with 31 in a serious condition.
The disaster is the worst railway accident to have taken place in Spain since 1944.