Francisco Jose Garzon, one of the two drivers at the helm of the Spanish train that derailed, killing at least 80 people, allegedly bragged on Facebook that he liked to drive fast.
According to screengrabs shown on Spanish TV and on news websites, Garzon posted a picture of a train's speedometer at 200 km/h.
Underneath the photo he allegedly wrote: "What joy it would be to get level with the police and then go past them making their speed guns go off. Ha ha!"
One of his friends wrote in response to the photo, which was posted in March last year: "Woah you are going so fast, braaaaaaake." Garzon allegedly replied: "If I went any faster, they'd fine me."
The page has since disappeared.
The report came after a Spanish court said one of the drivers of the train was under police guard in hospital, although it is not known which driver is being held.
Official sources have said Wednesday's crash, near the city of Santiago de Compostela, in northern Spain was caused by excessive speed. The eight-carriage train came off the tracks, hit a wall and caught fire. It was one of Europe's worst rail disasters.
The Galicia regional supreme court said the driver was being formally investigated but not under arrest. He was in hospital but it was not clear what kind of injuries he had suffered.
Video footage from a security camera showed the train, with 247 people on board, hurtling into a concrete wall at the side of the track as carriages jack-knifed and the engine overturned.
One local official described the aftermath of the crash as like a scene from hell, with bodies strewn next to the tracks.
Newspaper accounts cited witnesses as saying Garzon shouted into a phone: "I've derailed! What do I do?"
The 52-year-old had been a train driver for 30 years.
El Pais newspaper said he told the railway station by radio that the train entered the bend at 190km/h. An official source said the speed limit on that stretch of twin track, laid in 2011, was 80km/h.
"We're only human! We're only human!" the driver told the station, the newspaper said, citing sources close to the investigation. "I hope there are no dead, because this will weigh on my conscience."
Investigators were trying to find out why the train was going so fast and why security devices to keep speed within permitted limits had not slowed the train.