Speed Restrictions for Track Buckling in Hot Weather - Reducing the Operational Impact Whilst Managing the Risks

During very hot weather, emergency speed restrictions (ESR) are applied to minimise the risk of track buckling and/or derailment. Commonly, passenger trains are restricted to 60 mph and freight to 30 mph, which is very disruptive to operations. These limits have been in place for many years whilst significant changes to rolling stock and infrastructure have taken place.

As climate change leads to more extreme weather events, such disruption will become more frequent, and this paper outlines the work completed using modern modelling techniques to challenge historic standards and confirm the appropriate speed restrictions during extreme hot weather events.

Using dynamic simulations, the relationship between the vehicle type and operating speed, lateral track shifting forces, and derailment risk on buckled track was reviewed. It found that modern multiple-unit passenger vehicles cause lower lateral track forces than traditional locomotive-hauled trains. Whilst heavy freight trains apply higher forces than passenger trains at a given speed, they usually run more slowly which mitigates the risk.

The operating cant deficiency was found to be a more important risk factor than speed. In most cases, hot weather speed restrictions are significantly slower than normal linespeed, with negligible cant deficiency. In these conditions, the influence of speed and buckle size on track forces and derailment risk is reduced.

An assessment of Network Rail buckle records from recent years indicated that over 75% of buckles occurred on track with a linespeed of 60 mph or below, where the typical 30/60 mph hot weather restrictions would not change the speed of passenger trains. High linespeed buckles are relatively rare and tend to be longer in wavelength and on straight track, representing a lower likelihood of derailment.

The key finding is that the 30/60 mph hot weather differential speed limit may be overly punitive for both modern passenger and freight trains. There is scope to increase the freight train speed to 40 mph, and multiple-unit passenger train speed to 70 mph, without an unacceptable increase in track shifting forces compared to traditional loco-hauled passenger trains at 60 mph. A further increase may be justifiable for certain types of train.