Rail temperature is the temperature of the rail as recorded by a rail thermometer at the site. In Northern India, the maximum rail temperature in summer is about 20°C above the maximum ambient temperature and the minimum rail temperature is about 2°C to 3°C below the minimum ambient temperature.
The track on Indian Railways has been geographically divided into four rail temperature zones as per the following criteria:
Modern Welded Railway Track 321
Zone I 40°C to 50°C
Zone II 51°C to 60°C
Zone III 61°C to 70°C
Zone IV 71°C to 76°C
The mean annual rail temperature (tm) is the average of the maximum and minimum rail temperatures during the year. The value of tm, therefore, depends upon the range of temperature. The maximum and minimum rail temperatures for one year should be recorded in order to determine the mean annual rail temperature, although a record of five years is preferable.
Rail thermometers are used for measuring the rail temperature. The different types of thermometers available on Indian Railways are the following.
(a) Embedded type
(b) Dial type
(c) Continuous recording-type
The most commonly used amongst these is the embedded-type thermometer (Fig. 17.4). This type of thermometer consists of an ordinary thermometer inserted into a cavity in a piece of rail head. The cavity is filled with mercury and sealed. The rail piece containing the thermometer is exposed to the same conditions as the rails in the track. The temperature recorded by the thermometer at that time is the rail temperature. It takes about 25-30 min. to record the rail temperature.
Fig. 17.4 Embedded-type rail thermometer