How did the Spain train crash happen and what will investigators be asking?

Audio file appears to point finger at control centre, but there are questions around signalling and technical issues

At least 69 people died when a high-speed train from Madrid to Seville hit another train on the same track from behind near Toledo on Wednesday morning.

Both trains had been brought to a halt earlier when the signalling system failed. The driver of the state-owned Spanish train, a Renfe AVE service, was told by controllers to restart the journey, but crashed into the train in front, run by the private operator Iryo. The front carriages of the AVE train and rear of the Iryo train were destroyed and others derailed.