How to manage track in a heatwave
July 18-19’s record heat has made running the railway a real challenge. I’ve written before on managing track in hot weather (RAIL 856), but here is a broader look at how operations and infrastructure cope (or don’t).
Q: Why should we be concerned about heatwave conditions on the UK rail network?
A: Probably the number one problem is that trains much over a decade old will often have air conditioning that can barely provide any cooling when the external temperature exceeds 30°C.
Things are no better for staff on platforms or in buildings without sufficient air conditioning.
In addition, train traction systems, electronics and even brakes can overheat - in some cases, trains have to stop or won’t start.
Infrastructure problems certainly have a major impact. Track buckles resulting from unrestrained thermal expansion of steel rails are probably the best known. As a rule of thumb, rails become 20°C hotter than the surrounding air. For Network Rail infrastructure, the highest-quality track - fully welded flatbottom rails on concrete sleepers on fresh, well-profiled ballast - has a ‘critical rail temperature’ (a ‘take action’ temperature) of 59°C. Add 20° to 40°C and you can see why this is a problem.
Older overhead line electrification equipment (OLE) can sag as contact and catenary wires expand, increasing the risk of a train’s pantograph pulling them down. Even newer, auto-tensioning systems had a limiting design temperature of 38°C until 2012, at which point it was lifted to 40°C.
As we increasingly digitise train and traffic control systems, server operating temperatures become critical; shutdowns potentially knock out whole routes.
Extreme heat can desiccate and shrink earthworks, resulting in rough rides and sometimes in sufficiently deformed track that a line needs to close.
Lineside fires from dry vegetation and even from overcooked track or lineside components are a serious threat.
Lastly, the ability of staff to monitor high-risk areas or respond to failures will also be affected.
Q: What are the criteria/ considerations for closing whole parts of the network?
A: Infrastructure age and condition are the key metrics. Where air temperatures reach or exceed 40°C, even our newest track and OLE isn’t designed to operate normally. Where older systems exist, such as ‘unstressed’ switches and crossings or fixed-tension OLE, this means widespread speed restrictions or closures.
Q: What alternatives are possible?
A: Painting rails white at weak points (RAIL 856) can help slow how quickly they heat up, and reduce their temperature by perhaps 5°C. This can be enough to keep the railway open, even if trains have to run more slowly.
Track buckles are a dynamic phenomenon and occur under trains, so slowing speeds reduces the likelihood of a buckle and the consequences should one occur.
The same is true for overhead wires. Slower trains are less likely to cause a dewirement and make
less mess if they do pull the wires down.
Q: Why has the ECML been closed and not other lines?
The ECML has a significant proportion of older switches and crossings, as well as long-term track-quality issues thanks to geology. Also, the likelihood and severity of a dewirement is high.
However, closures and speed restrictions have not been isolated to the ECML.
Q: In the past, railways used jointed rails - should we return to these?
A: Jointed track is actually at higher risk of buckling than welded track, as the gaps between rails required in each joint close up in extreme heat. Even in lessextreme conditions, there are 320+ joints per mile of doubletrack railway to maintain - joints require lubrication, tightening (or loosening) and the expansion gap between rail ends needs setting quite precisely to manage thermal longitudinal forces.
Combined with the inherent configuration of jointed track as generally lighter and weaker, any lapse in maintenance results in significantly higher risk of buckling than with welded track, even before temperatures exceed design limits. Most buckles that NR records are in jointed, not welded, track.
Q: What do other places do and what possible future mitigations are likely to become more common?
A: Heat is an issue everywhere, and in many cases, the effects and mitigations are more substantial than in the UK.
Many countries have to re-stress their track twice a year, which is enormously costly and disruptive - on a network as busy as ours, this is simply not feasible. Italy paints almost all rails white, but again this is costly and has limited effectiveness with temperatures as high as we’ve just experienced. A higher ‘stress-free temperature’ can help, but we would see more rail breaks in winter.
Countries such as the Netherlands spend enough on infrastructure renewals that almost no older, less-resilient track remains in regular operation, reducing the need for restrictions.
By building new infrastructure such as High Speed 2, engineers can design out many of the failures we have just seen, for example by using slab track and minimising lineside equipment.