Don't fall for leaf delays

I'm taking a look at the autumnal excuse of leaves on the line and what the rail industry is doing about it in this #rail101 article

The problem

Leaves when they fall onto rails and get crushed can generate a very slippery layer.

How slippery? Typically it'll be 3-15 times less grip than a clean dry rail. Leaf contamination on rails is just like black ice on roads and pavements. A virtually invisible hazard to drivers with the same effect on stopping distance.

Aside from the operational issues of starting a train and the hopefully obvious safety issue of stopping it too there are also 3 more hazards that arise.

• Wheel flats - Not all trains have anti-lock brakes. A steel wheel sliding on steel rail can quickly generate a flat. When rotating that wheel turns into a hammer hitting the rail every revolution causing noise, damage and even in extreme cases breaking the rails.
• Wheel burns - Similarly a wheel spinning on the surface of a rail when trying to start a train moving can also damage the rail surface. Sometimes this damage is not obvious to see but can result in rail defects or even failure in time.
• Signalling failures - The sticky liquidised leaf layer can insulate trains electrically from the track. Some signalling systems rely on electrical contact between train and track to detect where the train is. No detection can result in delays or even safety issues in some cases.

The cost

In the UK, typically over 250,000 minutes of railway delay are attributed to low rail adhesion each year with some years being 40% higher.

The annual cost of low rail adhesion in the UK is estimated to be ￡355 million!

Leaf contamination isn't only a UK issue - many other countries experience similar issues too.

The cost of low adhesion isn't always just financial. Below is an accident where low rail adhesion was likely a significant factor in two trains colliding on entry to a tunnel. The interim accident report is linked here, (long read) if you want more detail.

Photo from Rail Accident Investigation Branch report

The solutions

With such a significant issue as you'd expect, there has been a lot of work to try to resolve and mitigate leaf contamination. Here are a few ways the issue can be tackled.

Remove lineside trees

Prevention is better than cure, No trees = no leaves. As you might expect , not a popular option with environmentalists and not without significant cost too - estimated to be in the region of ￡1000 per tree and there are hundreds of thousands of trees near the track. Some would argue that lack of lineside maintenance in the UK for past decades has seen trees encroach closer than ever before exacerbating the leaf-fall problem. Whilst this is the most robust method as it eliminates the issue entirely - cost and ecology factors mean it isn't popular in the UK. It should be noted that other countries have significantly clearer linesides, with the host of operational benefits that brings.

Remove the slippery layer

Currently the most widely used method to do this is via water jets aimed at the track from special rail vehicles. In the UK that is known as the Rail Head Treatment Train (or RHTT as the railway loves it acronyms). This jets water onto the rails at high pressure, (~22,000 psi), to blast the leaf residue off of the rails. The pressure is so high that the vehicle needs to be in motion before the water sprays are activated to avoid damaging the rail! It sprays around 5.5 tonnes of water per hour when at full speed.

Here is a picture of it in action - you'll note it is probably the dirtiest rail vehicle you'll see in service due to the leaf-laden spray covering the vehicle as it travels.

Photo from Geof Sheppard, CC BY-SA 4.0

For those wanting their rail video fix, you can it in action here

However, spraying water isn't a perfect solution to solving poor rail adhesion as water itself plays a crucial role in creating the slippery problem in the first place. So other techniques have been trialled and tested as alternative options

Lasers, Plasma and Dry Ice have all shown recent promise at removing the leaf layer from the rail head. A great summary article from Rail Engineer can be found here here (10 minute read).

Make the layer less slippery

If the friction between wheel and rail can be improved despite the layer being present, then that would give both operational and safety improvements.

The Rail Head Treatment Train doesn't just blast water at the rails it can also apply adhesion modifier to the rail head as it passes (often known as Sandite). It typically applies 1 litre of adhesion modifier per mile. The adhesion modifier is a mix of anti-freeze binder (to make it a liquid, able to stick to the rail), sand (to provide grip for train wheels) and steel or iron shot to break through contamination layers to help the track circuits work correctly.

Other more manual methods and equipment also exist to apply friction modifiers usually containing sand to the rail head for extra treatment or where the RHTT isn't able to be scheduled.

Another long-established way to to improve the wheel/rail friction is by the use of sand applicators on in-service rail vehicles. Either manually operated by the driver, or automatically activated on detection of wheel slide, these dispense dry sand via compressed air through tubes placed in front of the wheels at strategic positions. There has been recent work through the ADHERE programme from RSSB to optimise sanding with double variable rate sanders achieving half the stopping distance of conventional sanders in slippery conditions.

Photo from Ricardo Rail working on variable rate sanders and best practice.

Lastly in my list a slightly more surprising method of improving friction. Adding more water to the wheel/rail interface. Low rail adhesion is experienced under damp conditions rather than fully wet rail so adding water to the wheel/rail gap can actually improve adhesion.

There are probably other methods to solve this slippery problem that I've missed so please let me know below.

Live with it

Can we learn to live with low adhesion rather than fixing it? Arguably each autumn we already do. We amend timetables to cope with the increased times taken to accelerate and brake trains safely and the drivers adopt much more cautious driving techniques. This impacts on journey times and indeed capacity in highly trafficked routes so is far from ideal but it keeps trains running - albeit slowly and sometimes with delays.

The insulating properties of this leaf layer can also effect signalling systems based upon track circuit detection which has obvious safety implications. Other signalling techniques don't depend on wheel/rail electrical contact so this adds to the list of reasons why alternative train detection methods may be preferred when signalling upgrades are undertaken.

Lastly as it is an intermittent issue both in location and timing, accurate forecasting and modelling also has its place to minimise the impact on passengers, freight and indeed drivers.

The end bit

As we enter the leaf fall season in the UK and elsewhere in the Northern hemisphere, spare a thought for both drivers, track workers and rail operators as they cope with the very real issue that is leaves on the line. It is not an excuse nor an easily solved issue.

I hope you've enjoyed this article - if I've missed any vital issues then please let me know below. If you like it then share with others take a look at my other #rail101 content and feel free to follow me too. If you'd like help from me or my employer with your current rail challenges, then please do get in touch.