Destressing of Rails

Rails are made out of steel and are exposed to natural environment  through out  its life time in the railway track.This means rails get exposed to diffrent temperatures during the day and night.As we all know metal gets elongated when exposed to high temperature and gets contracted in the case of clod temperature.Due to this reason compressive and tensile forces get developed in the rails as they are fixed to the sleepers restricting their free movement with the themperature.There is a temperature where there is no forces in the rail and its called rail nutral temperature.If we can fix the rail and maintain it at this nutral temperature there will be no stresses on the rail and it will be ideal condition for the track.Practically this is out of reach and we can only minimize the tensile and compressive forces in the rails by laying it at a mean temperature.
Rails in Hot Weather

In hot weather conditions compressive stresses develop in the rails and as a result they get buckled.

Rails in Cold Weather
In cold weather conditions tensile stresses develop in the rails and as a result they get break. 

Buckling of rails and rail Break are two major reasons for derailing of rails.

The purpose of destressing is to avoid above two sinarios and provide a safer track for train movement.Destressing  for Hot weather and Cold Weather is done separately during two periods of the year.Part of the rail is cut and weld in hot weather case whereas a new peice is add and weld in the cold weather case.

Mean Temperature and Temperature Range for Destressing In Hot Weather.
To determine this mean temperature a detailed servey is required where we have to record rail temperature with time .Mean temerature depends on country to country and from region to region. With this data we can obtain the minimum and maximum rail temperatures and hence the mean rail temperature.In a tropical country this mean value is around 37C .We get the temperature range by adding 5 units to the higher side which is 37+5 = 42C.Here the plus side is preferd as it resembles tension in the rails which  avoids buckling.

Destressing at Site

In simple words we have to

remove the rail from elastick clips 

alllow its free movement with the tempertaure 

and fix it back

within the given temperature range .