LWR and CWR
What are the common lengths of rails?
The most common length for BG rails is 13m(42’8”) although double-length rails (26m, 85’4”) are also seen. Welded rail sections are of two types: Short Welded Rail or SWR which consists of just two or three rails welded together and Long Welded Rail or LWR which covers anything longer. (In the past, there was a distinction made between LWR and Continuously Welded Rail, or CWR, based on the length—in CWR, the total length was 0.75km or more. The term ‘CWR’ is no longer used although you may still find it in old documents or painted signs.)
LWR is typically any length larger than twice the breathing length, which allowed at the end of the welded rail section which is free to expand or contract as the temperature changes. (Beyond the breathing length, the rails do not move because of the resistance of the fasteners and the sleepers and ballast.) The breathing length varies with the temperature range, the sleepers, and the type of rails, but is typically 10m or less with concrete or steel sleepers. The expansion range of the rails is reduced with the steels of higher tensile strength, such as the 90UTS and 110UTS steels, allowing longer welded sections to be built.
With welded sections, the maintenance and safety problems of having rail joints with fishplates, etc., are reduced but welded rail also calls for more precise provisioning of distressing/pretensioning to account for thermal expansion, etc. SWR with three-rail welded panels results in 28-30 fishplated joints over the distance of a kilometer, which is the source of the commonly heard (and beloved of railfans) clackety-clack rhythm of the wheels.
LWR is usually formed from panels of 10-rail or 20-rail length welded using flash but welding at specialized plants (Meerut, Gonda, etc.). The welded rails are transported on special rail flat wagons which have end unloading chutes. LWR and CWR are also formed by in situ welding of the rails using alumino-thermic welding (also known as thermite (thermit) welding). In this, the highly exothermic reaction of aluminum with ferric oxide (provided as a plate called thermite) results in temperatures of around 2500C and the reduction of the ferric oxide to elemental molten iron that then helps form a weld.