Dependant on the joint maximum gap, the railway engineering science is defining the concepts of short rail (the one which, if installed correctly, theoretically will not develop any significant thermal axial force within the normal limits of the rail temperature) and long rail (that rail length which will cause the joint to close before the maximum rail temperature AND to open to the maximum gap before reaching the minimum rail temperature – for this rail length it is normal to develop axial thermal forces, compression or tension).

For free thermal expansion track superstructure, where the joint resistance and the track longitudinal resistance are ignored, the fishplated joint gap thermal variation can be shown as an inclined line (see below), in direct (linear) relation to the rail temperature. That will not be the case for the restrained thermal expansion superstructure – but this will be the subject of another article.

When managing the jointed track in hot weather (NR/L2/TRK/001/mod14) it is essential to establish at what rail temperature the joint will close. This is allowing the maintainer to estimate if there is a real risk for the rail axial forces to reach a critical limit – usually defined as the Critical Rail Temperature (CRT). Once the rail axial compression forces are reaching this critical limit the track buckling can occur.

The Joint Closure Temperature (JCT) for free thermal expansion superstructure can be calculated using this formula:

where:

• JCL is the joint closure temperature
• Tr is the current rail temperature
• ΔL is the current joint gap – usually the average value from a set of joint measurements
• α  is the steel expansion coefficient
• L is the rail length in metres

Since the joint closure temperature is inversely proportional to the rail length, the values computed for one rail length can not be linearly extrapolated to a different length. For each rail length the maintainer should compute the closure temperature dependant of the average measured gap.

Applying the formula above for a measured 8 mm joint gap, at 10°C rail temperature, the Joint Closure Temperature (JCT) will be:

• for 30 ft rail, JCT = 86°C. This means the joint will still be open at the maximum rail temperature. A normal behaviour for a short rail.
• for 45 ft rail, JCT = 61°C.This means the joint will still be open at the maximum rail temperature. A normal behaviour for a short rail.
• for 60 ft rail, JCT = 48°C.
• for 90 ft rail, JCT = 35°C.
• for 120 ft rail, JCT = 29°C. The joint is closing too early and, as the temperature grows further above 29°C, axial compression forces will develop that might endanger the stability of the track.

These JCT values are only theoretical and  presume the linear expansion of the rail, without the presence of any resistance force, either at the joints or of the track longitudinal resistance. Dependant on the way these resistance forces are mobilised along the track, the real joint closure temperature (JCT) will be a few degrees lower (or sometimes higher) than the one calculated as explained here.

References

• BSI BS 11:1985. Specifications for railway rails. British Standards Institution.
• NR/L2/TRK/001/mod14 (2012). Inspection and maintenance of Permanent Way. Managing track in hot weather. Issue 6. Network Rail. UK.