After the accident which occurred at around 8.42pm (18.42 GMT) on Wednesday the driver of the train told officials from the government of the northern Spanish region of Galicia that the train had been travelling at 190kph in an 80kph-zone.
"I have derailed (the train). What am I going to do? What am I going to do?", Spain's national broadcaster TVE reported a second driver as saying by telephone.
Both drivers were uninjured in the accident which affected a train travelling between the Spanish capital of Madrid and the city of Ferrol in northern Spain.
Those drivers are now receiving the full support of the driver's union, SEMAF.
Meanwhile, the President of Spain's national railway company RENFE says the train involved in the accident had been inspected on Wednesday morning.
The line where the accident occurred had "a security system" and the train involved had "passed inspection" on Wednesday morning, Gómez-Pomar told Spanish radio station Cadena Cope.
The Renfe President wouldn't be drawn on the causes of the crash but said the train's black box was "already in the hands of the judge".
"The accident is being investigated," Gómez-Pomar said.
Miguel Ángel Cilleros, Secretary-General for Transport for Spain's powerful UGT union said the accident was likely to have been caused "by a number of faults".
He told Spanish fee newspaper 20minutos it was unlikely there was a single cause.
While stressing the train's black box had yet to be opened, Cilleros said problems with the train network's braking system may have been a determining factor.
On Thursday, Spanish Prime Minister Mariano Rajoy visited the scene of the country's worst train crash in 40 years.
Rajoy, a native of the Galician town of Santiago de Compostela in northwestern Spain where the disaster occurred, is also due to visit victims in hospital.
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Spain will observe three days of official mourning for the victims of a train accident, Prime Minister Mariano Rajoy said on .
"Today I will sign a decree declaring three days of official mourning in all of Spain," he told reporters in his hometown of Santiago de Compostela where the accident occurred.
Spain's King Juan Carlos and Crown Prince Felipe, meanwhile, called off their public engagements on Thursday in a sign of mourning for the victims of a train crash that injured more than 140, a spokesman for the royal household said.
One Brit was injured in the accident according to the UK's national broadcaster the BBC.