Spanish derailment: Possible causes
Initial reports suggest that Wednesday night's train crash - one of the worst train crashes in Spanish history - may have been caused by excessive speed, though the official investigation is only just beginning.
El Pais reported that one of the two drivers on board said he was doing 190km/h (118mph) as he took the bend, where the speed limit is just 80km/h.
Security camera footage which shows the Madrid to Ferrol train hurtling around the bend immediately before the impact supports the theory that the train was going much faster than it should have been.
One of the train's drivers has been placed under formal investigation.
The head of Spanish rail company Renfe has told a radio station that the train had passed an inspection that morning, according to a report of the interview on AFP news agency.
The Renfe boss said as far as the company was aware there had been no technical problems with the train.
Attention is therefore likely to focus on the safety systems in place on the track to prevent speed limits being exceeded.
Sim Harris from Rail News told the BBC he was baffled as to how this could have happened.
"Modern trains have got so many systems on board to stop 'over speed' of this extreme kind," he said.
"The ability of the driver to break the rules in this way is very limited indeed by the on-board systems of the train."
The most modern train safety systems use equipment on the track and within the driver's cab to replace traditional signals and control the speed and movement of the train automatically.
With a system such as the European Train Control System (ETCS), a driver would not be able to break the speed limit.
While parts of Spain's rail network - including a large section of the route the train had travelled from Madrid - do have the ETCS in operation, the curve where Wednesday's derailment took place relies on a less sophisticated safety system known as ASFA.
ASFA - Anuncio de Senales y Frenado Automatico or signal notification and automatic braking - relies on a series of beacons to communicate with the driver's cab - so does not have the constant communication of ETCS.
The system gives audio and visual warnings to the driver if speed limits are surpassed, and will step in and brake the train if there is no response from the cab.
Rail expert Christian Wolmar, author of Blood, Iron and Gold, says that there will have been some sort of warning system in place.
"We don't know whether this failed or whether the driver didn't heed a warning," he said. It's also possible that the driver was confused by moving between the two different systems, he added.
"However the driver would have route knowledge," he said, pointing out that all drivers would have been trained extensively on a section of track before taking charge of a train carrying passengers along it.
Philippa Oldham from the Institution of Mechanical Engineers said accident investigators are likely to be looking at a number of factors which could have caused the accident, including the role of the signalling and speed advice system, as well as the role of the driver.
They would also look at whether breakages or vandalism damage to the track or the train contributed to the derailment.
"Rail travel remains one of the safest ways to travel, with far fewer deaths and injuries than other forms of transport such as car travel," she said.
"The UK also has the joint-safest railway safety record in Europe."
The train which derailed was a Renfe class S730 - which can travel on both the Iberian gauge tracks used by most of Spain's railways, and the standard gauge tracks on which the high-speed Ave services run.
The Alvia service which had departed Madrid at 15:00 local time had an engine unit at each end, and eight carriages.
The type of train involved came into service within the last two years. As well as being able to travel on both gauges in use in Spain, it is powered by a hybrid unit which can run off both electricity and diesel. This means it can be used for high speed services that start off on AVE lines and then continue on to towns and cities not served by the AVE network.
The train would have left Madrid on high speed AVE track, before moving onto standard Iberian gauge track at Olmedo.
At Ourense the train would have rejoined the AVE network, travelling towards Santiago on a newly constructed section of track which has the advanced train control system.
However the derailment occurred at the point where the high speed track transitions to using part of the older railway network. Some experts are asking whether this transition could have been part of the problem.
The accident took place on the A Grandeira curve, which is just after a tunnel and follows around 80km of more or less straight track.
A video posted on El Pais's website shows the curve as the train emerges from the tunnel.
Spanish journalist Miguel Murado told the BBC that there had been concerns about the bend since the line opened two years ago.
"People who travelled in the train felt that it was dangerous that the train had to go from 200km/h to 80km/h in just a matter of seconds," he said.
"They felt that was a very difficult manoeuvre for the driver to execute."
The BBC's transport correspondent Richard Westcott says Spain has led the world in building its high speed network over the past twenty years, and generally has a good safety record.
This is backed up by figures from the European Railway Agency, which put Spain 18th safest out of 27 countries in terms of passenger deaths per kilometre travelled, over the period 2006-2011.