Last Wednesday's accident involved a train run by state-owned operator Renfe with 397 passengers on board and a stationary train run by private operator Iryo, which was carrying 147 passengers.

A reported issue with the automatic signalling system - a computer failure - had forced all trains to halt on the affected stretch.

Leaked audio recordings published in Spanish media soon after the crash revealed conversations between train drivers and a control centre just minutes before the disaster.

The three recordings of conversations between Madrid's control centre and two train drivers appear to show that the Renfe train was given clearance to move even though the other train had stopped on the track, about 25km (15 miles) ahead.

In one, a control centre operator is speaking to the driver of the Renfe train, which was having some technical difficulties.

After some exchanges, the control operator tells the driver: "Continue your journey and then proceed under [Lateral Signalling Block], at normal speed with normal operation."

The driver replies: "At normal speed?" To which the controller answers: "Yes."

The Lateral Signalling Block is a back-up system used when the automatic signalling or data transmission system fails.

The driver of the stationary Iryo train says in another audio recording that communication with the train-control system has been lost, and it is not moving.

The train "is stationary in front of the entry signal at Urda", the driver tells the control centre operator. After a series of exchanges, the driver is then told to "resume the journey" normally.

The Iryo driver then contacts the control centre again, according to a later recording, saying they cannot start the train. "The train won't allow me to control the [security] system. That's why I am still stopped in front of the entry signal," they say.

The latter recording suggests that the controller had been notified that the Iryo train remained stationary as the Renfe train approached.

Iryo said its carriages were "struck from behind" by the other train.