Francisco Jose Garzon Amo, 52, refused to answer police questions Friday from his hospital bed, and the case was passed to the courts.
He was taken to a police station yesterday after being discharged from hospital and will appear today before a judge who will decide whether to press formal charges.
Under Spanish law, a suspect can be detained for a maximum of 72 hours before being heard by a judge.
The train was said to have been travelling at more than twice the speed limit on a curve when it was flung off the rails on Wednesday and slammed into a concrete wall, with one carriage leaping up onto a siding.
More From This Section
Regional authorities now say that 78 passengers died and 178 were injured in the accident.
It was Spain's deadliest rail accident since 1944 when hundreds were killed in a train collision, also between Madrid and Galicia.
Eight foreigners were among the dead -- a US citizen, an Algerian, a Mexican, a Brazilian, a Venezuelan, an Italian, a national of the Dominican Republic and a Frenchman.
Spanish media published photographs of the man they identified as Garzon Amo after the crash, with blood covering the right side of his face.
The driver should have started slowing the train before reaching a bend that train drivers had been told to respect, the president of Spanish rail network administrator Adif said Saturday.
Daily newspaper El Mundo, citing sources close to the investigation, reported yesterday that the driver was speaking on his mobile telephone at the time of the accident.
And El Pais, citing unidentified sources in the investigation, reported that the driver, while still trapped in his cab, told railway officials by radio that the train had taken the curve at 190 kilometres (118 miles) an hour. That is more than double the 80 kph speed limit for that section of track.