Rail Thermal Force (CWR)

Rail thermal force (CWR)

The axial thermal force in continuous welded rail (CWR) is F = E·A·α·ΔT, computed from the elastic modulus of the steel E, the rail cross-sectional area A, the coefficient of thermal expansion α and the temperature change ΔT relative to the neutral temperature. Continuous welded rail — rails welded into strings hundreds of meters or kilometers long, doing away with the joints and with the classic 'clickety-clack' of older trains — gives a far smoother, quieter and longer-lasting track, but brings one critical problem with it: thermal expansion is restrained by the fastenings. Since the rail cannot change length, a temperature swing builds up a huge internal axial force: compression in hot weather (with the dreaded risk of buckling — lateral misalignment of the track into snaking curves that can derail trains) and tension in cold weather (risk of brittle fracture of the rail or of the welds). The unnerving detail is that the force does not depend on the length of the rail — only on the section and on ΔT — and can reach hundreds of kN even in a modest rail section. Track engineers handle this by installing CWR at a carefully chosen neutral temperature (stress-free or rail laying temperature) in the middle of the thermal range expected in the region, so the compressive extreme in summer and the tensile extreme in winter stay balanced and within safe limits. Computing this force is essential for the safety of modern permanent way, for setting the neutral temperature and for designing the expansion joints at the ends and at other critical points. Enter the elastic modulus, the section area, the expansion coefficient and the temperature change.