El Pais is reporting they think it was a controller error,

https://elpais.com/espana/2026-10-28/por-que-han-chocado-dos...

An automated system[*] had failed earlier this morning, so controllers were working manually.

[*] https://en.wikipedia.org/wiki/Linienzugbeeinflussung ("LZB")

[added translation]: "The LZB safety system, which controls Spain's high-speed rail network, malfunctioned on Wednesday morning, causing about eight trains to come to a halt in the area. The control center then switched to manual control and ordered all trains to resume operation. However, the Iryo, for reasons that remain unclear, was unable to start and remained stationary. And that is where human error came into play, according to these initial hypotheses."

Every year that passes, Japan's record of zero fatal high-speed train accidents since the Shinkansen was inaugurated in the 1960s becomes more and more incredible, with 10 billion passengers and counting.

(Excluding suicides/people falling on tracks, that is. And that one poor student in 1995 who got stuck in the door of a departing train and was dragged to his death.)