Why did Francisco Jos? Garz?n, a train driver with 30 years' experience, hit a bend at 190 kilometres per hour when the speed limit was 80 km/h? Did he ignore the automated warnings? Or did his train's alert system fail at a critical time?
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. Garz?n 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 Garz?n 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.
To find out, investigators are now retrieving data from the train's electronic systems - a process that must be performed carefully to prevent critical data in the damaged systems being overwritten. This could take two weeks.
"There is a lot of data that can be downloaded from various traction control computers on a train - and that should tell the investigators if the train was doing what it should have done," says Kemp.
While ERTMS works well in general, it does have vulnerabilities which researchers are attempting to address. Researchers led by Gianmarco Baldini at the European Commission's Institute for the Protection and the Security of the Citizen in Varese, Italy, have developed a system that can detect radio jamming and interference of the GSM-R signal and, within 1 second, inform train drivers and track controllers (International Journal of Critical Infrastructure Protection, doi.org/dpfmht).
