© Marcio Machado/Zuma/EyevineAt about the point where the crash happened, one safety system switches to another
An inquiry is under way into the derailing of the packed train, which killed 79 people in Santiago de Compostela, north-west Spain on 24 July. Garzón has admitted to "confusion" over the train's speed and, though freed on bail, is facing the prospect of 79 charges of negligent homicide.
One focus of the investigation will be the fact that the crash took place at a point where one safety system hands over to another - from one that controls the train's speed to one that does not. On high-speed sections, the European Rail Traffic Management System (ERTMS) intervenes wirelessly to ensure a train slows down if alerts are ignored.
Crucially, ERTMS cuts in if its alerts are ignored. It does so using GSM-R - a robust railway version of the GSM standard used by cellphones to communicate with the cell towers.
"ERTMS has all sorts of measures that prevent trains going over speed and will eventually be fitted over the whole route from Santiago to Madrid," says Roger Kemp, a safety-critical systems engineer specialising in railway technology at Lancaster University, UK. "But it is not a finished project."
This means that, 4 kilometres from Santiago de Compostela, on a slower, bendier section of track that snakes through the town, ERTMS has not yet been fitted. Instead, an older Spanish-developed system called ASFA advises the driver of the necessary safe speeds. But ASFA can only intervene if the driver does not respond.
"The driver only has to acknowledge that they have seen the speed advisory by pushing a button - otherwise the system will apply the brakes - but you don't have to comply with that speed under ASFA," says Kemp.
Spanish TV station Antena 3 says that Garzón told some witnesses immediately after the crash that he was not able to slow down to the necessary 80 km/h before the sharp curve - but it is not known why.
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