Reported by BBC World
It was approaching 08:30 (07:30 GMT) on a normal Wednesday. Two high-speed trains were travelling south from the Spanish capital, Madrid, to Seville.
One, run by state-owned operator Renfe had 397 passengers on board. Ahead of it on the same line was a train of Iryo private operator with 147 passengers. A reported issue with the automatic signalling system - a computer failure - had forced all trains to halt on the affected stretch.
Then the impossible happened, Spain's transport minister said, as the Renfe train was ordered to proceed in "error".
More from the original report
It crashed into the back of the other train between municipalities Mora and Orgaz, about 140km (87 miles) south of Madrid.
Some of the passengers were asleep, some had already started their working day on laptops - and others scrolled through their apps on their phones.
The journey was interrupted with an announcement that there was congestion on the tracks. This lasted a few minutes - for some, a bit longer for others.
Tangui Binier, 22, was in the Iryo train, the front carriage, number eight. His father was in number seven.
Their train was one of six to have been ordered to stop at 08:30. A few minutes later, "the train started speeding", Binier told the BBC, "then it came to a sudden halt".
Baffled, he looked out of the window and saw "wires on the left side come down, and then there being just a lot of smoke behind us."
Spanish journalist Roberto Gómez was dozing on the high-speed Renfe train which had stopped more than 25km (16 miles) away from Binier's.
It was a brief stop. A few minutes later, the train began to move again, he explained, and then the crash happened.
He did not know that at 08:36 his train had been ordered to proceed with the journey and some 17 minutes later the "truly unbelievable" happened, Spain's Transport Minister Óscar Puente said.
"The impact was terrible," Gómez told Onda Cero radio. "You can imagine, the carriage is completely destroyed. All the suitcases are all over the wrecked carriage."
Gómez suffered only minor…