Leaves on the Line is Actually Not a Terrible Excuse

Dear reader, I am sure that, if you live in the UK, “leaves on the line” is an excuse you have heard for the late running of a train. I am also sure that, if you have any sensory capabilities at all, you will have noticed that it is autumn, and, therefore, there are leaves falling from the trees.

I will admit, it is not immediately obvious that these would cause a problem for trains. After all, trains tend to be large and quite heavy, whereas leaves are neither of those things. It is also true that a railway (at least in this country) tends to be a substantial construction, so much so that the track is often referred to as the “permanent way”. The question then arises as to why, if this is all true, leaves on the line should be a problem.

To answer this question, we must first look at what makes trains so efficient, and to do that, we need to look at cars. You see, a car has rubber tyres, which run on a tarmac road. Both of these surfaces are rough, and as a result, the friction between them is high. Even when it is wet, the contact between tyre and road is not lubricated since the tyre is designed to channel water away from the contact (in all but the most extreme cases), and your car still grips the road. The other thing about car tyres is that they are quire wide and, therefore, the contact area is large, so even if some of it is slippery, chances are a good tyre will find grip somewhere.

Trains are very different. Wheels on trains have steel tyres, which are very smooth, running on a steel rail which is also very smooth, leading to very low resistance. This is great in normal conditions, since the rolling resistance is very low and you can transport large loads with relatively little effort. However, as the wheels are quite thin, and there is not much “spring” in them, the contact area for each wheel is about the size of a pound coin. Suffice to say, it is very easy to get a train wheel to slip, with very little of the rail needing to be slippery.

Now, it might surprise you to learn that the problem is not leaves per se. Dry leaves are, in themselves, not that slippery. Unfortunately, as leaves get crushed between the wheel and the rail, a leaf mulch is created. Add in a little water from rain, and perhaps some grease and other contaminants,  and you now have something akin to washing-up liquid on the rails, which the train will find it near-impossible to grip. Given that a mature tree might have some tens of thousands of leaves, and that there are 20,000 miles of track in the country, this is no rare problem.

If the train cannot grip the rail properly, it cannot accelerate nearly as fast, and, equally, cannot brake properly either, without the wheels locking. This is potentially very dangerous, and so trains tend to travel at lower speeds, further increasing journey times and delays. Of course, when the poor passengers are told, there is not time to explain the problem properly, hence the “leaves on the line” excuse.

Of course, railway engineers have not been sitting idle while the problem persists. Most passenger trains in the UK are now equipped with a system called WSP, or Wheel Slide Protection, which is a bit like your car’s anti-lock brakes and traction control rolled into one. However, while this does mitigate some of the problem, the train will still be slower if WSP is constantly cutting power to avoid slipping, or varying the brake to find grip. It does at least reduce the damage done to the wheel (and the rail) by slipping – which can be very expensive indeed.

Another solution is the use of (and I apologise for the all the acronyms) RHTTs, or Rail Head Treatment Trains. These (very noisy) trains go around the network every autumn, spraying a substance called “sandite” onto the rails, which is a sort of gritty paste-like substance, improving grip dramatically. This is only a partial solution though, and the rails must be treated regularly if it is to be effective.

The obvious solution is to just get rid of the trees next to the railway. This, like so many things in life, is much more complicated and expensive than it may seem. The decision to fell trees cannot be taken lightly, as the roots of trees may be helping to stabilise earthworks the railway depends on. Furthermore, tree felling near railways has come under fire from environmental groups such as the Woodland Trust and Greenpeace (not helped by some very dubious reporting in the Guardian newspaper). For tress not technically on railway land, landowners must be consulted, which can lead to its own problems, and, apart from anything else, felling a tree next to a railway line must be done with great care to avoid it falling onto the railway itself.

One charge that could be levelled against the railway is that “this never used to happen”. And for once, the people saying this are, by and large, correct. In the days of steam, there were few trees next to railway lines, and consequently, there were few incidents of leaf mulch on the line. Why was this?

Well, steam engines tend to produce sparks (after all, they rely on a large fire to work), and these sparks, when hitting dry vegetation, can lead to fire. Lineside fires were something that the old railways were understandably very keen to avoid, and so they made sure that linesides were kept perfectly manicured. Any tree that sprouted in this environment would have to come to terms with being on fire before or being chopped down long before leaves on the line became a problem.

However, with steam trains being phased out in the 50s and 60s, lineside fires became less of a problem. The new diesel and electric trains did not produce sparks, and so the linesides could be left alone (which was quite convenient because it saved British Rail a lot of money). In fact, for a while, British Rail banned steam engines (even those run privately) from its lines entirely, partly to prevent lineside fires. Vegetation growing on the now-not-well-looked-after linesides has unfortunately resulted in the current situation.

The supreme irony at the end of all this is that this article was itself delayed, though not through leaves. I have been rather busy, as mentioned in the public service announcement, but I have also begun writing for other places, which has rather taken up my time. Do not fear though – I will still be writing on here, just not with the frequency to which you may previously have become accustomed.

It only remains for me to wish you an excellent day, and ask politely for you to pass this on to whomever might find it useful and/or interesting.







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