Technique
Unconstrained large sections of steel will shrink as the temperature drops, and expand as it increases. Since the ends of CWR are fixed, the rail will experience tensile stress in extreme cold and can fracture if it grows too great. In extreme heat it will experience compressive stress, and if that force grows too strong the rail will buckle.
Imagine a one kilometre length of CWR track that has some stress in it. The rail is restrained by being fixed to the sleepers/ties. Also imagine a one kilometre length of rail lying on the ground next to the track. In a hot summer, the unconstrained rail lying next to the track is going to expand, while the fixed rail will experience compression. The unconstrained rail will expand to become longer than one kilometre. Likewise, in extreme cold, the unfixed rail will shrink and become shorter than one kilometre, while the rail that is constrained to one kilometre in length will experience tension.
Obviously, there is a temperature at which the length of the rail fixed to the track is equal to the length of the unfixed rail. This is known as the "rail neutral temperature". Note that rail neutral temperature does not actually measure the temperature of the rail. Instead it measures stress, because rail neutral temperature is merely the ambient air temperature where the fixed length of rail has no compressive or tensile stress.
When working with rail (laying new track, repairing track, or changing the sleepers/ties), this rail neutral temperature can be induced, even if the ambient temperature is different, by adding stress to the rail.
The act of stressing rail induces the proper rail neutral temperature so that there will be no fracturing or buckling at the temperature extremes. Since environmental extremes will vary, there is no universal rail neutral temperature. In the UK all rail is stressed to 27°C (81°F), the mean summer rail temperature). US standards range from 35 to 43°C (90 to 110°F), depending in large part on expected temperature range over the course of a year.
Stress is frequently induced after a piece of rail has been removed due to a defect, or when an IBJ (Insulated Block Joint) is to be replaced. In these cases the old rail is marked, and clips securing it to the sleepers/ties are removed. The start point of the new section of rail is then cut. At this point the rail should shrink (creating a gap) due to the stress being lost. The technician then calculates how much stress has been lost and needs replacing. A second cut is made and the old rail removed. The new rail is fitted and welded at one end. A stressing kit is then fitted at the end still to be welded. The kit is clamped to the rail and it pulls the gap together, leaving enough space for a second weld. Once the rail is stressed to the appropriate temperature the weld is made. After setting the weld the stress kit is removed.
The stress or neutral rail temperature of track can change over time, particularly under heavy traffic conditions. Besides resetting the rail neutral temperature when making repairs, rail testing is often employed to attempt to calculate the rail neutral temperature of a particular track to determine if prescriptive measures are needed.
Read more about this topic: Rail Stressing
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