Ongeza ya ballast vs ballast less track bed
How do they deal with expansion and contraction in continuous welded rails?
Probably they use joints after several welds.
CWR therefore requires less maintenance than jointed track, and will remain in accurate alignment for longer. But if it is continuous, with no expansion joints, how does it cope with thermal expansion in hot weather?
Its laid it’s in tension by hydraulically stretching it before cutting to length and welding to the previous length. Sufficient tension is applied to achieve a ‘Stress Free Temperature’ (SFT) of 27 degrees C (i.e to relieve the tension by expansion the rail would need to be heated by the sun to 27C, whereupon it would neither be in tension or compression).
The rail temperature at which the expansion stress has the potential to start causing buckles on normal, plain track in good condition is 32C above stress free temperature. So, an SFT of 27C means that rail temperature has to reach 59C before mitigation measures are made (such as blanket speed restrictions). And a rail temp of 59C in high summer, with blue sky, (worst case) requires a shade air temperature of 41C, something which has never been recorded in this country.
The recent heat wave is the first widespread and prolonged period of high temperatures since 2006. Both the 2006 episode and the extreme heat wave of 2003, when 38.5°C became Britain’s highest recorded temperature ever, disrupted Britain’s rail network through buckled rails and the related speed restrictions. Although this year’s temperatures are slightly lower than those of the previous episodes, the impact on rail operations has been significant, as witnessed by the thousands of passengers stranded at London Waterloo during the evening rush hour on 15 July 2013, because a rail buckled in a junction area. Part of the problem is that track defects develop unobserved during cooler years and emerge quite suddenly when a heat wave occurs. Then, lessons and skills have to be learnt afresh!