What are the types of rail failures?

Why Rail Failures Matter

A rail failure is more than a maintenance cost: a broken rail can derail a train. Understanding the failure types is therefore a core competence for track engineers and for buyers who specify rail quality. Rail failures are usually classified by their mechanism - fatigue, wear, rolling contact fatigue, or brittle fracture - and each type has a different cause, appearance and remedy.

Fatigue Failures

Fatigue is the most dangerous failure mode because it can progress internally without visible surface signs. Fatigue cracks start at material defects, welds, bolt holes or corrosion pits, and grow under repeated wheel loading until the remaining rail section can no longer carry the load. Typical outcomes are transverse fractures across the head or web, and longitudinal cracks along the rail. Fishplate bolt holes are a classic initiation site, which is why hole-edge cracking is specifically monitored on jointed track. Prevention relies on clean steel with low inclusion levels, controlled welding, and ultrasonic or eddy-current testing at scheduled intervals.

Wear Failures

Wear removes material from the rail surface. It appears in three forms. Head wear flattens the running surface and changes the contact geometry; side wear on the gauge corner is driven by curve negotiation; and corrugation - the periodic short-wave undulation of the running surface - develops on tangent track and sharp curves under particular vehicle-track resonance conditions. Wear is managed by choosing harder rail grades (such as head-hardened or bainitic rails) in curves, by lubrication on sharp curves, and by periodic grinding, which restores the head profile before wear accelerates.

Rolling Contact Fatigue (RCF)

RCF is the family of surface-initiated defects caused by repeated wheel-rail contact stress. It appears as shelling (spalling) - surface or near-surface material flaking off the head - and as squats, the dark, comet-shaped surface cracks that grow downward and can break the rail if left untreated. RCF is driven by contact pressure, traction, and the presence of contaminants such as water in the contact patch. Grinding is the primary countermeasure: removing the thin cracked surface layer before cracks grow deep enough to become dangerous.

Brittle and Low-Temperature Fracture

Most rail steels are susceptible to brittle fracture at low temperature, particularly if the steel contains defects or if the rail has been damaged by impacts. Old wrought iron and early steel rails were especially prone to sudden breakage in winter. Modern rail steels are specified with toughness requirements (for example, impact or fracture toughness limits in the standard) to reduce this risk, and rails subject to very low service temperatures may be specified in improved-toughness grades.

How Failures Are Detected

Ultrasonic testing: the standard method for internal defects such as transverse fractures and weld flaws, run by hand or by testing vehicles.

Eddy current and magnetic particle testing: for surface and near-surface cracking, including RCF.

Visual and geometric inspection: for wear, corrugation and profile changes, supported by track measurement cars.

Rail grinding cycles: scheduled profile grinding both corrects wear and removes the RCF layer before deep cracks form.

What This Means for Rail Procurement

When specifying rail, the failure record matters. Clean steel with tight inclusion control, a documented heat treatment process, and verified toughness at the service temperature all reduce the probability of fatigue and brittle failure. For curves and heavy-haul tonnage, a harder grade pays back through fewer wear and RCF defects. Buyers should ask the supplier for the steelmaking and rolling process description and for test data on cleanliness, hardness and toughness, not only for the tensile strength.

FAQ

What is the most common cause of broken rails?

On modern continuously welded track, the most common causes are rolling contact fatigue defects such as squats and shelling growing into transverse cracks, weld defects, and internal fatigue initiated at inclusions or bolt holes on jointed track.

Can grinding completely prevent rail failure?

No, but it is the most effective single measure against RCF. Grinding removes the cracked surface layer and restores the head profile, so defects cannot grow to critical depth. It must be combined with lubrication, drainage and ultrasonic testing.

What is a squat on a rail?

A squat is a rolling contact fatigue defect that appears as a dark, roughly comet-shaped mark on the running surface with a shallow crack inside. If grinding does not remove it in time, it can grow into a transverse break.

Why do rails fail more in winter?

Low temperature reduces fracture toughness, so a rail with an existing crack can break at a lower load than in summer. Thermal contraction also increases tensile stress in continuously welded rail. This is why winter is the peak season for rail breaks on older lines.

Are harder rails always better against failure?

Harder rails resist wear and RCF better, but hardness alone is not enough - the rail must also have adequate toughness and the track must manage contact stress. An extremely hard rail with poor toughness can crack in cold weather, so grade selection must balance hardness, toughness and weldability.