Sharp bend in track outside Santiago de Compostela known as 'A Grandeira' is supposed to be approached at just 80km/h

The Alvia 151 train crash near Santiago de Compostela, which killed 80 people and left 95 in hospital, took place on a curve in the tracks that several rail officials, consulted since the disaster, have termed "problematic".

As it leaves the tunnel, the track goes down a short, gentle slope and then starts to form goes into a tight bend known locally as A Grandeira. By then, trains like the one that crashed on Wednesday, which can reach a speed of 250km/h, have been travelling in an almost straight line for more than 80km from the inland city of Ourense.

The sharpness of the curve is one of two reasons why the speed limit at A Grandeira is a mere 80km/h. The other is that the track on the approach to Santiago de Compostela station, unlike those on the rest of the stretch from Ourense, do not form part of Spain's independent, AVE high-speed train network.

They were laid in the days of General Francisco Franco's dictatorship, which ended in 1975. According to the daily El País, the old track was retained to avoid having to add to an already considerable number of compulsory purchase orders issued to facilitate the passage of the line.

The threat posed by the A Grandeira curve calls for trains to reduce speed drastically as they approach it. Indeed, when the high-speed route from Madrid to Ferrol was officially opened on 10 December 2011, some of the passengers on the inaugural journey reported being jolted violently as the train braked close to the site of Wednesday's disaster.

Though Spain was the first country to make use of the EU's European rail traffic management system (ERTMS), which automatically brakes trains exceeding designated speeds, the system was not in use on the Madrid-Ferrol line. It is governed instead by a Spanish safety system, called ASFA.

This is meant to stop trains altogether if the driver fails to respect the relevant signals. But it only receives information about the speed of trains from certain points along the line.

The most important of the questions left by the tragedy is why the Alvia 151 did not brake to 80km/h as it was meant to. Was it human error? Or a technical failure?

Much of the earliest evidence points to the former. One of the train's drivers, trapped in the cab, was reported to have given an account over the radio to officials at the station in Santiago de Compostela. He was quoted as having repeated over and over: "We're human. We're human."

The driver was said to have put the train's speed at the exit from the tunnel at 190km/h, then at 200km/h, before returning to his original version.

Both of the men in the train's cab escaped with minor injuries. Four minutes away from its next stop at Santiago de Compostela, the Alvia 151 was just five minutes late.