Evaristo Iglesias told Antena 3 television that he and another person accompanied the blood-soaked Francisco Jose Garzon Amo to a stretch of flat ground where other injured people were being laid out, waiting for emergency services to arrive.
"He told us that he wanted to die," Iglesias said.
"He said he had needed to brake but couldn't," Iglesias said. He added that Garzon said "he had been going fast."
It also aired television footage of Iglesias working beside the wrecked train to help other survivors.
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Garzon has been released from the hospital and is in police custody on suspicion of negligent homicide. He is expected to give testimony to an investigating judge later today, though he exercised his right to remain silent when police tried to interview him.
The judge will also have access to information contained in the train's "black box," which is similar to those found on aircraft.
Authorities have pointed to speed as the culprit, and officials have said that the brakes should have been applied four kilometres before the train hit the curve.
Investigators must determine if Garzon failed to apply the brakes or whether it was a technical failure.
Iglesias was among survivors and witnesses who began to give evidence to police today.
Meanwhile, authorities said forensic experts have identified the last three bodies among the dead. They did not reveal the names of the dead, but said today that all of the families had been notified.
A large funeral mass is planned for tomorrow, and the prime minister and royal family are expected to attend.