MADRID (AFP) - The driver of a fast-moving train that flew off the tracks in northwestern Spain, killing at least 80 people, was unable to brake in time, newspaper El Pais reported on Friday.

The investigation into the cause of the nation's deadliest rail disaster since 1944 has focused on a possible problem with the brakes and speeding by the driver, the newspaper said.

The accident happened on Wednesday evening as the train entered a sharp curve known as A Gandeira about 4km from the station in the northern city of Santiago de Compostela.

Dramatic video footage from a security camera showed the train slamming into a concrete wall at the side of the track as the engine overturned, piling carriages on top of each other in a smouldering wreckage of mangled steel.

The train was reportedly travelling at twice the limit on a section of high-speed track that has a speed limit of 80 kilometres per hour.

"The railway warning systems detected that Francisco Jose Garzon Amo, the driver of the Alvia train that departed Madrid, was travelling at 190kmh when it should not exceed 80," El Pais wrote.

"The driver acknowledged that the alarm went off in the control panel and he tried to brake but was not able to avert the tragedy," the newspaper added.

El Pais said on Thursday that the driver stated immediately after the crash that he had been travelling at 190kmh at the time of the derailment.

"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 derailed.

Police are waiting to question the driver, who according to state train company Renfe has more than a decade of train driving experience.

He is in hospital under police surveillance as he undergoes treatment for light injuries sustained in the accident.