Rail defects

Wear and fatigue are unavoidable, but you can slow them down significantly

Rails are subjected to severe stresses from heavy freight traffic, high-speed long-haul trains and the constant braking and acceleration of rail cars in light rail traffic. In addition to natural wear, these stresses also become evident in the form of various types of rail defects.

Rail defects can be roughly categorized into abrasion (loss of material through friction), deformation, fatigue and machining errors. The maintenance procedure used – grinding or milling – depends not only on the type of defect, but also on its severity. The amount of material requiring removal varies depending on how deep the damage goes. The rail should be measured beforehand in order to calculate the exact amount of material that needs to be removed at each point on the line.

Preventive maintenance at regular intervals, such as High Speed Grinding (Verlinkung) maintains the rail and stops defects while they are still just beginning to form. This prolongs the rail’s service life considerably while significantly reducing the cost of maintaining the rail during its lifetime. In the future our grinding machines will be fitted with measuring and testing equipment that measures the condition of the rails as the machines pass over them. The data collected forms the basis of our revolutionary Smart Maintenance Concept which allows you to manage your customized maintenance “on the fly” via an App.

The following is an overview we have put together of the most common types of rail defects.

Head checks

Head checks are probably the most common type of defect that occurs on rails – fine, closely-spaced cracks caused by rolling contact fatigue and appearing predominantly on the rail’s running edge. They form on the surface as upward-facing open cracks which, if not removed from time to time, deepen further into the rail’s interior where they join up and in later stages cause break-outs and weakening of the rail until replacing the rail becomes the only option. Head checks constitute an increased safety risk if the rail’s degradation is not halted because the rail could ultimately break apart completely.


Wave-like deformations on the rail surface occur through rolling contact on straight sections and especially in bends, but also as a result of incorrect machining. Corrugations are the main cause of increased rail noise. The vibrations caused by corrugations put the track system and the vehicles under constant strain – damage to trains and the track bed is inevitable over the long term.

Corrugations are divided into three categories according to wavelength: short-pitch corrugations (short), slip waves (medium) and long pitch corrugations.

Other forms of fatigue damage can occur from the wheel constantly running into the wave peaks. This type of defect is known as a “Belgrospi”: clusters of cracks that form on the wave peaks. If left unchecked, these develop into squats (see the next section).

Slip waves occur on the inside rail of tight bends. They result from wheel set slippage caused by the difference in rolling radii between the wheel on the inside and the wheel on the outside of the bend.

Long pitch corrugations are rarer, but when they occur they are not always easy to differentiate from errors in the track geometry.


Squats are typically isolated defects, but they can sometimes occur in series. A squat is distinguishable as a dark-colored depression, and in more advanced stages of progression squats exhibit a V-shaped or semicircular crack opening towards the running edge. There are various hypotheses as to their cause; some sources speak of a lateral compression of the metal, while others see squats resulting from pre-existing damage such as corrugations, indentations or grinding errors (martensite formation from bluing the metal while grinding). The occurrence of squats has increased considerably over the last few years; some sources regard squats as the most frequent and serious type of rail defect.

Skid spots

Skid spots result from friction between the rails and the slipping (spinning) wheels of traction vehicles as they accelerate. The wheel spinning on the rail can deform the rail profile, and the heat produced causes a structural change in the metal, which then hardens in that area. Skid spots can be easily distinguished from squats, for example, because they always appear at the same time opposite each other on the left and right rails.


Indentations on the running surface result from hard foreign objects on the rail being run over. This can occur for two reasons. If it is a single, punctiform defect that does not reoccur, then it was caused by an impurity being pressed into the rail by the wheels. If a foreign object has become stuck to a wheel, however, a new indentation is produced with each revolution of the wheel. This results in regularly spaced defects over a longer section of track. Deformations of this kind can ultimately cause cracking.


One symptom of advanced-stage rail defects are areas where material breaks out of the metal surface. This occurs when head checks are not ground away or cracks in the rail’s interior are not milled off, allowing them to get bigger and with time join together to form weak points in the rail. If wear limits have not yet been reached, break-outs several millimeters deep can still be milled out (Link zur HPM / MPM). If break-outs are so deep, however, that there is no longer any point in removing the surface material, replacing the rail is the only option.

Machining errors

It’s not only the wear and tear of rail traffic that shortens the service life of rails, incorrect machining methods can also damage the metal. “Aggressive grinding” is one such example. The increased grinding pressure and rotation speed of actively driven grinding wheels is supposed to remove as much material as possible, but the rail often overheats as a consequence and bluing or martensite hardening is the result.

A further problem arises when not enough material is removed during machining and as a result rail defects are not completely rectified. Residual defects are masked, which results in further damage. As an example, new corrugations will form quickly in an area where corrugations are not ground or milled off properly.

A “sub-layer grind” should always be carried out regardless of the type of defect being removed. Here material is removed to just below the depth of the defect, which is the only way to ensure that localized damage and any hardening or cracks are removed completely.