Chemistry of leaves on the line

Along with “the wrong kind of snow” the station announcement about the delay to trains because of “leaves on the line” causes much derision and eye-rolling among disgruntled passengers, who often think it’s a made up excuse. The phenomenon is, however, a real one and can be a hazard when trains slide past platforms, and very dangerous when they can’t stop at red signals. Steel wheels on steel rails just don’t get on with leaves.

Michael - They make the rails really slippery. So the train tries to brake, it will just slide over the rails and it can miss stations. And because of that, we have to change timetables in the autumn. And the other thing they can do is when they build up in thick layers, they can electrically insulate the train from the track. And that means that the signals don't work properly, because they rely on conduction between the train and the track to tell where the trains are in the network. We tried to strip back the system and make it as simple as possible, while still making a very low friction.

That was Sheffield University’s Michael Watson. And in research published this week, he and his colleagues think they’ve figured out what’s going on between the leaves and the rails to cause the low friction conditions. By brewing up some sycamore leaf tea and one by one eliminating potential culprit chemicals, they think that tannins - which, by coincidence are also in the tea so many of us enjoy - are to blame. Michael told Katie Haylor what they did, and firstly how they ended up pointing the finger at tannins...

So instead of using leaves with all their complicated parts, and instead of using real world contamination, we use leaf extracts. So this is essentially tea that we've made out of Sycamore leaves. And we found that we could get extremely low friction with that tea, so that already rules out anything that doesn't come out in the tea. So all of the things in the cell walls, and then we looked at that extract and we removed things to see which ones when we removed them, caused the friction to increase.

Katie - So you're trying to detective-like investigate the culprit by finding which ones it isn't.

Michael - Yeah, that's right. Yeah. Chemically in our setup, we have this leaf extract, which is slightly acidic, and that will dissolve the iron, which came from these test samples. But in a real railway would come from the rails. When that dissolves, the iron ions are floating around in the solution, and they get grabbed by these tannin molecules. And because each ion can get grabbed by multiple tannin molecules, and each tannin molecule can grab multiple ions. By the end of this, you end up with this big, loosely bonded, cross-linked network. And that's what really causes the low friction.

Katie - And this is the black, kind of, gooey stuff that's causing the hassle, right?

Michael - Yeah. So when these tannins grab the ion, they also leave the solution and they turn into a black precipitate, solid, that's floating in the solution.

Katie - Okay. So the theory is, you've got leaves falling on the track. These leaves are what, degrading a bit? And there's some sort of acid there. That dissolves iron from the tracks, and tannins from the leaves grab onto these iron ions, making this low friction producing black stuff.

Michael - Yeah, that's exactly right.

Katie - Great. What's going on at a molecular level though? Why does this black stuff have such low friction?

Michael - We think the cause of the low friction is because it's very thin. It's almost as thin as water. So it's not viscous like honey, but when you compress it on a sort of micro scale, when the surfaces are contacting, it gets compressed very hard into this hard black plastic. And that means that you don't get the viscous drag that you would get if you're in something like honey, but the surfaces can't touch each other, because every time they get near to touching, it turns into this harder plastic. So you end up with the sort of, best of both worlds way of reducing the friction.

Katie - How comparable is your experiment to what's actually happening?

Michael - We've shown one thing that does cause this very low friction, but we can't take the results from this and say that we're definitely showing the only thing that can possibly cause low friction. You have to take it with a pinch of salt, but it's the first time that anyone has linked a particular chemical to the very low friction that they get on the rails.

Katie - What are the options for actually preventing this from happening? Because I guess that's the goal, right?

Michael - Yeah. So either you can protect the rails from corrosion, which you might want to do anyway. Or you can put your own chemical down that will grab the iron ions better than the tannins can, so that when the iron ions come out, they don't get grabbed by the tannins. They get grabbed by your molecule instead, and then they can't make this big complex. Something we would like to keep working on. But as the labs are all closed now it's hard to get anywhere with it now.

Katie - I see. So we need to watch that space. But it's interesting you mentioned the tracks, because I was wondering is the iron the thing that's the problem? Would it at all be feasible to just change the track material in at risk places?

Michael - Track materials have been around for a very long time and people have spent years and years optimising. So the materials we have at the moment, are quite complicated materials, and swapping it to something like nickel, you'd really be taking a leap into the dark. And it'd be very, very hard to justify doing.

Katie - What about the trees though? Does it depend on the type of tree, and is felling an option? Cause there's an environmental cost to that. Right?

Michael - Well, felling is something that happens anyway. It's the method of last resort that Network Rail have. If they need to sort out a black spot. With this research, we can talk about which trees contribute the most. So we might be able to reduce felling where it happens with it still being there as a last resort.

Katie - And is that a case of which leaves contain the most tannins?

Michael - Yes. Yeah. That should be it. Well, we know that oak and sycamore are very high in tannins. They're also very common in problem areas. So that's what we use, sycamore, in the study, but there's also a lot of anecdotal evidence surrounding oak.