New tech to stop rail delays

Shooting pellets of dry ice to remove sludgy leaves from train tracks
12 October 2021

Interview with 

Roger Lewis, University of Sheffield




We’re well into Autumn now which often ushers in a perennial problem: leaves on the line and delayed trains. It seems bizarre that something as mundane and fragile as leaves can bring public transport to a halt in the 21st Century, but it is a serious problem. Tannins in leaves react with iron in the rails to produce a super-slippery surface that the train wheels slide over. Thankfully, mechanical engineers at the University of Sheffield may soon be riding to our rescue with a way to make this commuting catastrophe a thing of the past. Sally Le Page reports…

Train announcment - We are sorry to announce that the 1822 train to Cambridge has been delayed due to leaves on the line. We are sorry for any inconvenience caused.

Sally - Ugh, the announcement we all dread to hear. But leaves? Why are high-tech trains in the 21st century thwarted by leaves? Roger Lewis explains.

Roger - Every autumn as the leaves are falling from the trees, these are crushed by the passage of trains and you get some chemical reactions taking place in the interface that create this black material that adheres to the railway track very well. You'd find it very hard to scratch it off even if you were poking the track with a screwdriver or a metal blade, which means that trains struggled to brake safely, but also to accelerate away from stations which causes delays and makes passengers unhappy.

Sally - So the leaves are chemically bonded to the track. It's not just like when I'm walking on the pavement and the leaves are all wet and mushy, it's a bit slippy. It's more than that.

Roger - It is more than that.

Sally - And how do we currently deal with them?

Roger - So at the moment, network rail use what are called railhead treatment trains, which fires water at the rail head under very, very high pressure. The pressure is so high that if the train stopped and the water kept going, it could possibly cut through the rail.

Sally - Cut through solid steel?

Roger - Yeah.

Sally - That is incredibly high, and must use a lot of water as well.

Roger - It does. This is one of the problems with the trains. They can only run a certain distance before they need refilling.

Sally - So your solution involves using dry ice. What is dry ice, other than the thing you put in fancy cocktails to make them go all smoky?

Roger - The pellets we use are basically solid carbon dioxide. Now that sounds very bad. We don't like carbon dioxide at the moment, but actually all the carbon dioxide we use is recaptured from other industries that are putting it out into the atmosphere. So most of it comes from the fertiliser industry. Basically you have to compress it under very high force so that then it turns into a solid. Then you have to keep it very cold to keep it solid. So when it comes out of our machine and hits the railhead, it's at about minus 70 degrees centigrade.

Sally - How are you using dry ice to remove this slippy layer from the track?

Roger - We fire it onto the top of the railway track in a stream of air, which is actually going at supersonic speed. So you've got these pellets going at very high speed. Because they're cold they make the leaf layer brittle. And then because they're back in the atmosphere, they turn back into a gas. So you've got lots of little air explosions, almost, across the top of the railway track. So you've got three mechanisms there which are acting to break up and remove the leaf layer.

Sally - I'm imagining a machine gun firing continuously little ice pellets. Does it look anything like that?

Roger - That's kind of how it works, but that's not how it looks. It doesn't look as exciting as that, I'm afraid.

Sally - You've got these dry ice machine guns strapped to passenger trains. Can they just run as normal? Will this happen during the normal commute from London to Cambridge, or does the train have to be going particularly slowly for it to work?

Roger - That's a good question. At the moment, railhead treatment trains, for example, are designed to run at 60mph. A lot of passenger trains on local lines would typically run at that speed or a bit less. So we've obviously been targeting high speed operation. We were testing it on a full-scale wheel rail test at 60mph. We've run on track on a train at 40mph. And hopefully this autumn we'll be able to go a little bit faster. The good thing about cleaning on a passenger train is that hopefully we'll be able to stop the really heavy contamination building up. So if we can clean more frequently, we can do a lighter clean, which is obviously much easier at higher speed.

Sally - It's a bit like brushing your teeth often so you don't have to go to the dentist and get horrible treatments done.

Roger - Yes, quite.

Sally - And how long do you think it's going to be until we no longer have the dreaded announcement of "this train has been delayed due to leaves on the line"?

Roger - I'd like to say very soon. I think our systems will be underneath trains probably within a couple of years, but we can use it already on a railhead treatment train or road-to-rail vehicles. So we're already cleaning the track with it

Sally - And it can't come soon enough. Now, when is that next train arriving?


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