Railway experts explain how and why a rail can break

Regarding the train accident in Adamuz (Córdoba) and its causes, there are very few things that can be taken for granted at this time. Almost the only certainty is that it will take months to know what caused the derailment of an Iryo train on a straight line and, everything indicates, the subsequent impact of a Renfe Alvia train seconds later.

Despite this and despite the fact that Angrois railway accident (Santiago de Compostela) has already made it clear to us that these investigations entail a great effort of time and resources, information that points to one cause or another continues to be published.

Among this information that, for the moment, remains conjecture, the idea of ​​a fracture of the road has become relevant following the publication of an image in which three researchers are seen next to a broken rail. in the diary The Country This hypothesis is pointed out as the fact that focuses the investigation. ABC He claims that it is the cause of the derailment. RTVE He points out that investigators want to confirm if it was the cause or consequence of the train leaving the tracks.

The image, published by several media, is being used on social networks to defend that this is the real reason for the accident, accompanying video information in which strong vibrations from the trains in motion are observed. The latter, in fact, has been taken into account to lower the maximum speed to 230 km/h in a four points of the line between Madrid and Barcelona by Adif in what is considered the first really drastic measure after the accident in Andalusia.

But what causes a fracture in the road and what are the implications? Is it related to the vibrations of the trains we travel on?

A fracture in the road

The first thing to make clear is that in this article we try to explain how the bill can occur on a track, what its implications are and if it has any relationship with the vibrations we feel on trains. However, until now there are no official sources that confirm that the original cause of the Adamuz accident is this. The investigations continue and probably It will take months to know all the details.

The General Council of Industrial Engineers reminds us of the same thing, who emphasizes that “it cannot be stated without data whether the breakage is a cause or consequence. The investigation must be based on records, tests and metallurgical analysis. Not on images after the accident.”

With this in mind, they point out that “a stress fracture is a progressive break of the lane that is not produced by a single sudden event, but by the accumulation of tensions over time. Simply put, the rail supports millions of load cycles. If there is a weak area (defect, welding, microcrack), each train passage does not break the rail, but it degrades it. “There comes a time when the resistant section is insufficient and the rail suddenly fractures.”

From this entity they clarified to us that the vibrations we feel when we are traveling are not enough to derail a train. For this, one of the following scenarios must occur:

• Serious lane breakage.
• Severe loss of track geometry (alignment, grading, width).
• Structural failures in train elements (axles, bogies).
• Major obstacles on the road.
• Very unfavorable combinations of speed, geometry and undetected defects.

And they emphasize that “usual vibrations are expected in the design of both the train and the infrastructure. “High-speed rail systems work with very wide safety margins.”

This is confirmed to us from SEMAF (Spanish Union of Railway Machinists), who point out that imperfections in the track multiply when driving on them. “It is steel on steel,” they remember, and emphasize that the vibrations are a consequence of very small perfections in the track or the wheels that generate damage to their opposite. If the damage is on the track, it generates another imperfection in the wheel that multiplies it with each step cycle, generating the discomfort we feel on board.

The General Council of Engineers emphasizes that “it is not usually a safety problem. It is usually a comfort or maintenance problem (wheel or rail) and many vibrations are corrected by re-profiling wheels or rails, without touching the structure of the line.”

That is, when we feel these vibrations repeatedly and repeatedly It is not that we are passing through broken or fractured paths.. But it is possible that over time they end up being damaged to the point of suffering a stress fracture if appropriate measures are not taken.

Maintenance is essential

In this case, The road had been renovated last May with an investment that has reached 700 million euros. We cannot yet know if this was the origin of the accident, but the General Council of Industrial Engineers points to three possible causes that could cause the breakage of a track:

1. Rail manufacturing defects: Non-metallic inclusions. Internal microcracks. Steel segregations. They are rare, but possible, and that is why periodic ultrasonic tests are carried out.
2. Defective welds (especially aluminothermal): a poorly executed weld can generate residual stresses, poor alignment and/or internal microcracks. It is not common, but it is a known cause in railway engineering.
3. Fatigue from repeated loads: Each axis introduces vertical, lateral and longitudinal loads. At high speed, dynamic effects multiply those loads. If the rail is already “touched”, fatigue accelerates the breakage.
4. Thermal stresses on track without joints (the usual one today):

• The lane is “blocked.”
• Heat generates compression.
• The cold generates traction.

A combination of low temperature, residual stresses and previous defect can promote brittle fracture.

It must be taken into account that “the rail is one of the most demanding structural elements that exist. It is not rigid on its own, it is part of a flexible system. The steel of the rail has elasticity and the elastic fasteners allow small displacements. The ballast acts as a pseudo-damper system. In addition, in plate track, resilient layers are added under the rail.” All of this, experts point out, allows the rail to work with the following loads:

• Vertical flexion (train weight).
• Lateral loads (guidance in curves).
• Longitudinal loads (braking, traction).
• Thermal stresses.
• Fatigue (millions of cycles).

For all of this to work, the engineers explain to us, “the design seeks a balance, not maximum flexibility. It is not about eliminating bending, but rather controlling it, distributing stresses and avoiding stress concentrations. More flexibility does not always mean less risk,” they clarify.

Regarding the possibility that the track fractured under the weight of the train and not before it passed, they clarify that “if there is a previous complete breakage, it is normal for the first bogie to be affected. But it is also technically possible that the breakage begins under the passage of the train” as a consequence of this stress fracture.

The latter are mere conjectures about the breakage of the tracks which, as we say, nothing confirms that it was the original reason that caused the derailment of the Iryo train and its subsequent collision with the Alvia train.

Photo | Ministry of Defense

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