1 August 2013 15:44

In the week following the train crash at Santiago de Compostela, the reasons slowly came to light. Minutes after the crash we knew that the train had derailed and the driver was reported to have said it was travelling at around twice the line speed limit. Was this a speed that would have caused derailment?

Consulting Google Maps, the radius of curvature at that point seemed to be around 500 m. This is consistent with a speed limit of between 70 km/h and 100 km/h depending on how much the track is banked.

When Britain’s research into tilting trains started in the 1970s, one of the first questions was how fast trains could go round a curve without derailing. The maths are relatively easy (for the technically minded: ensuring that the vector addition of the weight and the centrifugal force stays inside the outer rail). But a test instils more confidence than a calculation, so a series of tests was undertaken by shunting an old coach round a sharp curve near Dover at ever-increasing speeds. This confirmed the maths and, at the speed predicted, the wheels on the inside of the curve came off the track; at a higher speed the coach rolled over.

Applying this formula to the Spanish train gave a derailment speed of around 170 km/h, which is what was reported. The train is designed to operate both on electrified lines and on non-electrified lines. For the latter, there are generator coaches at each end of the train to supply power to the electric motors. Looking at the video of the crash, one of these appeared to be the first coach to roll, which is to be expected as the centre of gravity would be higher with a heavy diesel engine in the coach.

Within hours of the accident the evidence was pointing to excess speed being the immediate reason for the crash, but why was the train going so fast? Next morning, on the BBC, a specialist journalist said that 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. The ability of the driver to break the rules in this way is very limited indeed by the on-board systems of the train." (Sim Harris, Rail News)

It turned out that this was only part of the story.

The accident was on the partially-complete, 424 km, high-speed line between Madrid, Santiago de Compostela and the Galacia coast. Construction on the north part of the line between Ourense and Santiago de Compostela, where the accident happened, was complete by December 2011. However, the Olmedo – Ourense section is not yet built.

Since the 19th century, the track gauge in Spain has been 1,688 mm, compared with 1,435 mm in most of the rest of Europe (including Britain, but not Ireland). New high-speed lines are being laid to the European standard, so trains can travel into France and thence to the rest of Europe. The 88 km section between Ourense and A Coruña has been designed to this standard but, until the connecting lines are complete, it has been laid with 1,688 mm rail spacing so it can be used by older trains.

Eventually the line will be fitted with the new European train control system, ERTMS, which is what the BBC interviewee referred to. However, because the track west of Santiago will be used by older trains, it is fitted with a system called ASFA (Anuncio de Senales y Frenado Automatico) which requires the driver to acknowledge they have seen a warning signal by pushing a button. The system does not however check the driver is slowing down. The transition between ERTMS and ASFA takes place a few kilometres before the accident site.

Many commentators have said that, even if the driver’s speed was not continually monitored, he should have seen the speed limit signs, known where he was, and slowed down. If he didn’t, he was to blame.

Lancaster University runs a part-time MSc in Safety Engineering for students in industry. In the module on human factors students study cognitive determinants of human reliability and situational awareness. In this accident we have to ask whether the driver really knew where he was on the route (or, in more academic language, was he suffering from a deficit in situational awareness).

The new line passes through difficult terrain and has 31 tunnels, totalling 30 km, and 38 viaducts. The last tunnel before Santiago ends less than a kilometre before the new line joins the old and goes into the 500m radius curve. A week after the accident it emerged that the central control room had phoned the driver as he was approaching Compostela. With the distraction of an administrative phone call, a changeover of the signalling system and a line forever running into and out of tunnels, would it be surprising if the driver had a situational awareness deficit?

Several years ago, the Head of Human Factors from the HSE Rail Division gave a seminar to the Safety Engineering course. She made a distinction between accidents where it was legitimate to blame the driver and those where the driver was doing his/her best to cope with a system that was set up to fail. When the inquiries into this accident are complete, we will know which of these is the more likely explanation in this case.