Like any infrastructure, the rail network can be impacted by external factors such as the weather. Met Office scientist, Alice Lake, is part of a team developing a new temperature model designed to help rail networks in the UK minimise these impacts.
The underlying infrastructure
In the past, the UK rail network was made up of tracks laid in separate panels, bolted together. But traditional track joints require significant maintenance; rail ends wear, and bolts need periodic checking and tightening. Therefore, as steel production and manufacturing processes improved, the lengths of the panels progressively increased until the 1960s when the first continuous welded rail was laid, eliminating track joints altogether. Today, continuous welded rail is used as standard across the UK network.
The weather impacts
Since the steel on railway tracks is essentially a very long piece of metal, it can expand or contract significantly in hot and cold weather. Therefore, to increase resilience, rails are pre-stressed to withstand the range of temperatures they’ll typically experience. The so-called ‘stress-free temperature’ (SFT) is the temperature at which the rail is neither in tension nor compression. In the UK, the SFT is 27°C. Other countries may choose higher SFTs – for example, in the US standard SFTs range between 35 and 43°C – which allows their rail to withstand hotter temperatures before expanding. However, this also means that their rails are more susceptible to brittleness and cracks in low winter temperatures.
If the rail temperature exceeds the range in which it is designed to operate, the steel can expand substantially, generating forces which push and pull the railway out of shape. Combined with the forces generated by trains running at high speeds along the tracks, this can lead to buckling. While rare, derailment caused by track buckling can have catastrophic consequences, so blanket speed restrictions are implemented across the UK network when the air temperature is forecast to exceed a set threshold.
Using science for informed decision-making
However, there isn’t a simple relationship between rail temperatures and air temperature. Therefore, at the Met Office, we’re currently developing a new rail surface temperature model, working with key partners to understand their needs. This model uses forecasts of local atmospheric weather parameters (such as air temperature, incoming shortwave and longwave radiation, and amount of precipitation) from numerical weather prediction (NWP) models and then calculates the incoming and outgoing energy at the rail surface. From this, we are able to produce forecasts of local rail surface temperatures. With accurate and precise knowledge of rail surface temperatures, speed restrictions and other preventative measures (such as painting rails white to reduce overheating) could be applied in a more localised and targeted way, minimising disruption to the network.
A changing climate
The latest UK Climate Projections (UKCP18) indicate that we can expect hotter, drier summers as our climate continues to change. Services such as the rail surface temperature model will therefore continue to be critical to ensure industry and business can remain resilient to current and future weather impacts.