The 52-year-old driver — one of two on the train travelling between Madrid and the northern Spanish city of Ferrol — was under police surveillance in hospital after the train accident late on Wednesday.
State train company Renfe said the driver was a 30-year veteran of the firm with more than a decade of train driving experience.
The train's data recording "black box" and other documents were passed over to the judge in charge of the investigation on Thursday.
Attention has so far centred on the driver, after media reports described him as a speed freak who once gleefully posted a picture on his Facebook page of a train speedometer going at 200 kilometres per hour (125 mph).
Below the photo he wrote the caption: "I am on the edge, I can't go faster or else I will be fined."
The man's Facebook page has since been taken down, but Spanish newspapers quoted another of his posts as saying: "What fun it would be to race the Guardia Civil (police) and pass them, causing their radar to blow up hehehe. What a huge fine that would be for Renfe."
Police originally intended to question him on Thursday but had to wait because he was still being treated for light injuries sustained in the crash on the outskirts of the pilgrimage city of Santiago de Compostela, a police source said.
The driver told authorities "I've messed up, I want to die;" Spanish news agency Europa press reported citing transcripts that make up part of the judicial investigation into the incident.
The El Pais newspaper, citing sources close to the investigation, said the driver stated immediately after the crash that he had been travelling at 190 km/h (118 mph) on a curve with a speed limit of 80 km/h.
"I am going at 190! I hope no one died because it will weigh on my conscience," he reportedly told supervisors over the radio while trapped inside the cab after the eight-carriage train flew off the tracks on a curve at 8:42 (1842 GMT).
The Galician regional government said 80 people died and at least another 100 were injured in the accident,.
The US Department of State confirmed that one of its citizens was among the dead and five others had been injured.
Eighty-three people were still in hospital, 32 of them in critical condition, including four children.
It is Spain's deadliest rail accident since 1944 when hundreds were killed in a train collision, also between Madrid and Galicia.
Of the 80 dead, 13 still had not been identified. Officials were holding off giving a complete list of the injured until everyone has been identified.
Renfe president Julio Gomez-Pomar Rodriguez told Cadena Cope radio station that it was too early to speculate about the cause of the disaster, but the Spanish secretary of state for transport, Rafael Catala, said excessive speed appeared to be the culprit.
"The tragedy that happened in Santiago de Compostela seems to be linked to excessive speed, but we are still waiting on the judicial investigation," he told radio station Cadena Ser.
Renfe said the train had no technical problems and had just passed an inspection on the morning of the accident.
Many of the passengers were thought to be on their way to a festival in honour of Saint James, the apostle who gave his name to Santiago de Compostela, an annual event that draws crowds of pilgrims to the town.
All festivities have been cancelled as Spain plunged into mourning for the victims.