(Repeats story from July 25 without changes)
By Andrés González and Julien Toyer
MADRID, July 25 (Reuters) - Why was the train going so fast?Did the driver fail to heed speed limits on a sharp curve? Didbrakes fail? What about the safety system meant to force thetrain or the driver to slow down if going too fast?
These are among issues investigators will look into afterSpain's worst train crash in decades, which left at least 80dead and 94 injured, 35 of them in serious condition.
A day after the crash, the driver of the train whichderailed on the outskirts of the northern Spanish city ofSantiago de Compostela was under police watch in hospital buthad not been arrested.
A judge in the Galicia region ordered police to question thedriver, Francisco Jose Garzon, as a suspect and also orderedthem to seize the black box of the train.
The 52-year-old driver was a 30-year veteran, said Renfe,the state train company. It has been widely reported that hetook a sharp curve with an 80-kmph speed limit at more thantwice that speed.
Many newspapers published excerpts from his Facebook accountwhere he boasted of driving trains at high speed. The accountwas closed early on Thursday.
The driver was not available for comment and Reuters was notable to locate his family or determine whether he has a lawyer.
Representatives of railway unions said it was too early totell whether the driver was to blame.
"Human error is always a possibility, and in defence ofhuman error what do we have, we have technology, that is what itis for ... but it is very difficult to know what might havehappened without, for example, hearing what the driver wassaying at the time," said Miguel Angel Cillero, responsible fortransport at union UGT.
While police and the judge were looking into potentialnegligence on the part of the driver, the Public Works Ministrylaunched a more technical investigation. Renfe and Adif, thestate track operator, began their own probes.
Security video footage showed the train, with 247 people onboard, hurtling into a concrete wall at the side of the track ascarriages jack-knifed and the engine overturned.
The impact was so strong that one carriage of the train flewover a wall and landed on an embankment several metres above.
Main story on the train disaster
Timeline on Spanish train tragedies
TWO SAFETY SYSTEMS
The train involved, made by Bombardier and Talgo, was aseries 730 that Renfe uses for its Alvia service, which isfaster than conventional trains but not as fast as the AVEtrains that criss-cross Spain at even higher speeds.
The train was built in 2007-2009, but remodelled in 2012 touse diesel.
The train is designed to operate on conventional andhigh-speed tracks that make use of two different types of safetysystems that are meant to regulate excessive speed.
On high-speed lines, trains use the European Train ControlSystem, or ETCS, system, which automatically slows down a trainthat is going too fast.
On slower lines, trains operate under an older system calledASFA, a Spanish acronym for Signal Announcement and AutomaticBraking, which warns the driver if a train is moving too fastbut does not automatically slow it down.
At the site of the disaster, just 3 km before reaching theSantiago de Compostela station, the train was passing through anurban area on a steep curve. At that point of the track, tworailway experts said, it uses the older ASFA safety system.
Professor Roger Kemp, a Fellow of the Royal Academy ofEngineering in Britain, said in an e-mailed comment: "As thedriver was leaving the high-speed line to join a much slowerroute before entering the station, there must have been at leastprominent visual warnings to reduce speed, if not audiblewarnings and an electronic speed supervision system."
Local media reported that railway union representatives hadquestioned whether a high-speed train should have been adaptedto run on a track with curves that had been designed forlower-speed trains.
A source close to ADIF said the safety system was apparentlyworking correctly and a train had passed an hour earlier with noproblems.
The train, packed with families visiting relatives andrevellers on their way to a major religious festival, was notrunning late.
It began its seven hour journey to the northern region ofGalicia right on time: at 15.00 CET on the dot. It crashed at20.41, two minutes before it was due to arrive. (Additional reporting by Kate Kelland, Teresa Medrano andElisabeth O'Leary; Writing by Fiona Ortiz and Sarah White;Editing by Peter Graff)