Rail101 - Training in the heat

As the recent UK heatwave fades, I take a little look at what challenges hot weather brings to railway infrastructure. I'm writing from a UK perspective but the same challenges face many global railways grappling with climate change or increasingly common extreme weather events.

I'll cover in my slightly longer article:

• Rail expansion
• Overhead catenary expansion
• Earthwork failure
• Control equipment / Electronics
• Fire

Here is a great 2.5min intro to the first two topics from Network Rail

Rail expansion

Steel expands when it gets hotter. A 1km piece of continuously welded steel rail 'grows' by around 1cm for every degree centigrade temperature rise. As rail temperatures can be 20 degrees above air temperature, (Think of how hot your car roof gets in the sun), there can be quite a length change to accommodate. To do this we 'stretch' the rail when we install it. As the rail expands with increasing temperature the 'stretch' is used up and eventually at a special temperature there is no stretching stress left in the rail. This is called the "Stress Free Temperature" or "Stress Neutral Temperature" which is referred to in the video above. Above this temperature the rail is in compression and if that compression force gets too high you can get this...

To resist track buckling we have a few tools in the box we can deploy.

1. We can stop the rails getting as hot, (Yes really!)
2. We can stretch the rails more during installation, or
3. Increase the stability of the track.

The first method is most easily achieved by painting the rails white to limit their temperature gain from the sun. (As an aside this is the reason the majority of vans used to be white - they stayed cooler in the sun before aircon became common.)

You can get a 5-10°C rail temperature reduction which certainly helps and white painting is routinely done in some countries such as Italy. In the UK white paint is usually restricted to around switches and crossings (which also suffer reliability issues when the rail temperatures are high due to the precise geometries needed).

Stretching the rails more during installation to achieve a higher stress neutral temperature also increases the track stress ('stretch') in winter. This increases the risk of rail failure, so a careful balance must be struck. (See my article ?"why rails fail more often in winter" for more info).

The last method, although the most robust fix, is hugely expensive and disruptive as it involves replacing conventional ballasted track with concrete slab tracks. It just isn't practical for the limited number of days disruption caused currently.

For completeness, one further option is to change the track stress (Stress Neutral Temperature), between summer and winter as some networks with extreme temperature fluctuations do, but I hope we won't see enough extremes of temperature in the UK to make that seem a sensible option.

Overhead catenary expansion

Turning our attention upward to overhead electrification wires for a moment the thermal expansion for copper is higher than that for steel. As most overhead electrification relies on tension to keep the contact wire at the correct height above the train, a tensioning system is designed to cope with a certain degree of temperature variation.

Different tensioning systems exist but weights dangling from a pole are usually the easiest for people to understand. As the temperature goes up the wire grows and the weights go down. If the weights get to the floor tension in the contact wire is reduced or lost entirely and the wire begins to sag.

Having 25,000 volts in the wrong place can certainly ruin a passengers day both as the train stops due to damaged wires or pantograph, is compounded by being in a sealed metal tube (train), by having no aircon (due to no power) and in the high temperatures that caused the issue to start with.

Electrification systems can be designed to work over larger temperature ranges by making the wire runs shorter, or tensioning towers taller - but this increases the cost, a deeply unpopular decision to take during the design stage.

Some electrification systems in the UK were specified with a maximum temperature of 38°C, which is now regularly exceeded. Last years heatwave delivered over 40°C temperatures (a record) and climate scientists predict more volatile weather in future. As I understand it current "new" UK electrification specifications are for 40°C which doesn't seem very future-proof to me.

Earthwork failure

Long periods of hot and dry weather alter the water content of the ground the railway is supported by. Inside railway earthworks, particularly those not built to modern standards, shrinkage can result from the loss of water. This can cause track alignment issues particularly where shrinkage is uneven. In severe cases subsidence can affect safe railway operations. Earthworks failures often require extensive remediation works to get traffic flowing normally again. Often it is said "Water is the greatest enemy of the railway" - However, sometimes the extended lack of it can cause trouble too.

Control equipment / Electronics

Pretty much all modern rail networks are controlled by electronics or electrical systems, and those systems are designed to fail-safe. So if something goes wrong and some component or equipment fails, then almost universally trains are commanded to stop moving to keep people safe. The disruption from signalling or traction power failures can be significant and cover a wide area.

Electronics and electrical components hate getting hot. A rule of thumb that says for every 10°C temperature rise a component lifetime halves. We know this and so we cool our server and relay rooms, so I ask why we have so many lineside cabinets painted dark green or grey? Surely white or light colours would be better to reduce solar gain and avoid the need for active cooling solutions as much as possible in the future.

Fire

Long dry spells and high temperatures can create conditions where wildfires can thrive, particularly where lineside vegetation management hasn't been kept on top of. Fires can vary in scale from the inconvenient to the disastrous. Whilst fires started by normal railway operations (excluding rail grinding) are thankfully rare, the long asset boundaries of the railway are not respected by a raging blaze and the impact on operations can be severe particularly where lineside equipment is damaged.

Did you like this?

This?#rail101?has been a little longer than most, but I hope you've enjoyed the read over a good cup of tea or coffee, or perhaps something cooler.

• Do you think I've missed another important heat related challenge?
• Do you have a top tip to beat the heat?

Comment to add more insight for the whole rail community.

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