Rail steel has a considerably higher carbon content (0.7-0.8%), and hence is more brittle than mild steel. A variety of stress concentrating defects in rails, combined with the alternating loads from the passage of traffic, can produce slowly propagating fatigue crack. When this crack attains a critical size it causes an almost instantaneous brittle fracture (Cope – 1993).
The fatigue crack in the rail comes from a variety of causes:

• load/unload cycle of the passing traffic
• manufacturing process – which can leave inherent defects in the rail
• weld defects
• corrosion

The high tension forces in the continuous welded rail during the cold season can increase the crack and even break the rail. When the break occurs, part of the rail tension force is released and the break opens. On both sides of the break the rail thermal stress is disturbed and a stress transition zone is formed (L in the drawing below).

The length of this stress transition zone is dependant of the track longitudinal resistance and of the temperature difference (ΔT) between the rail break temperature (T_break ) and the Stress Free Temperature (SFT).

The graph above shows the relation between the break gap, the track longitudinal resistance and this rail temperature difference. It also indicates, indicatively, the length, L, on which the rail thermal force is disturbed. This presumes that the rail break occurs within a fixed zone of CWR and not on a stress transition, where the thermal stress is irregular and potentially higher than the practically constant found in the central fixed zone of CWR.

References

Cope, G. H. (Ed.). (1993). British railway track: design, construction and maintenance. Loughborough, UK: Permanent Way Institution.