1. Introduction: The Science of CWR (Continuous Welded Rail)
UIC 720 defines the technical rules for the laying and maintenance of Continuous Welded Rail (CWR). Unlike historical jointed tracks, CWR eliminates expansion gaps (fishplates) to provide a smoother ride, reduce wheel/rail wear, and lower maintenance costs.
However, eliminating the gaps creates a massive physics challenge: Thermal Expansion. Since the rail cannot expand longitudinally, temperature changes generate immense internal forces (Compression in summer, Tension in winter). UIC 720 is the manual for managing these forces to prevent catastrophe.
2. The Holy Grail: Stress-Free Temperature (Neutral Temperature)
The core concept of UIC 720 is the Neutral Temperature (Tn). This is the precise temperature at which the rail has zero internal stress.
If the rail is laid or fixed outside of this calculated range, two major failure modes occur:Scenario Physical Phenomenon Risk (Failure Mode) Temp >> Neutral Temp
(Hot Summer)Excessive Compressive Force Track Buckling (Flambaj): The track suddenly shifts laterally, causing immediate derailment. Temp << Neutral Temp
(Cold Winter)Excessive Tensile Force Rail Breakage (Pull-Apart): The rail snaps, creating a dangerous gap in the continuity.
3. Laying and “Destressing” Procedures
Laying CWR is not just about fastening clips. It is a precise thermal operation defined in Chapter 7.
3.1. The Destressing Operation
If the ambient temperature during laying is not equal to the required Neutral Temperature (e.g., usually 25°C – 30°C depending on the region), the rail must be artificially lengthened or shortened. This process is called Destressing.
- Mechanical Tensors: Hydraulic jacks pull the rail to simulate the expansion required for the Neutral Temperature.
- Heating (Rail Warmers): In cold weather, the rail is physically heated to expand it to the target length before clipping.
3.2. Welding Techniques
UIC 720 specifies the methods for joining rails into infinite lengths:
- Flash Butt Welding: The preferred factory or on-track machine method. Uses high electric current to forge ends together. High strength, no foreign material.
- Aluminothermic (Thermit) Welding: Used for site closures and repairs. Uses a molten steel chemical reaction. Requires strict pre-heating and finishing tolerances.
4. Maintenance: The “Breathing” Zone
Even CWR has “Breathing Lengths” at the very ends of the track (near bridges or switches) where some movement occurs.
- Ballast Resistance: To prevent buckling, UIC 720 mandates a robust ballast profile (shoulders) to provide high Lateral Resistance.
- Creep Monitoring: Maintenance teams must monitor “Creep Marks” to ensure the rail is not migrating longitudinally over time, which would alter the Neutral Temperature.
Site Manager Note: Never disturb the ballast or release fastenings on a hot day (when rail temp > Neutral Temp + 15°C). Doing so significantly reduces lateral resistance and can trigger instantaneous buckling.
