Tannins in Leaves Cause Train Delays

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During autumn, we see leaves fallen from the trees around roads and train-tracks. Apparently, they are harmless but they can be a problem, especially, for the trains. Did you know that among different causes of train delays, the seasonal leaves fallen on the train-tracks is a main reason of train delays? This problem becomes severe in autumn globally. Recently, researchers at the The University of Sheffield have found one of the most probable reasons of this problem.

Why fallen leaves on the rail are dangerous?

Fallen leaves are crushed by the wheel-rail contact leaving a black layer. The actual composition of this black layer is unknown to date. Under wet conditions (e.g., during a rainy day) this black layer acts as a lubricant and reduces the maximum coefficient of friction between the wheel and rail to 0.01.  The mechanism by which this friction is reduced is not well-understood. 

High level of contamination present in the wheel-rail contact and complex chemistry of the leaves have complicated the problem. 

Rust on rail

What are the new findings?

First the authors produced the above-mentioned black layer in the lab. For this, they extracted the chemicals from naturally fallen Brown sycamore leaves. These extracted chemicals were reacted with Iron (III) chloride solution which produced the black layer as a precipitate. This black precipitate was then analyzed by various chemical characterization techniques. 

The obtained results have shown that the tannins present in the leaves are most probably the chemicals which generates the black layer when the fallen leaves are crushed by the train’s wheel on rail. Tannins are poly-phenols which are quite often present in plant leaves. It is known for many years that ionic iron forms a complex with tannins. The dissociated hydroxyl ligands on the on the tannins chelate the iron ions. Cross-linking occurs between tannin molecules at neutral pH. Such cross-linked tannin molecules act like loosely bonded hydro-gel. Upon wheel-rail contact, this hydor-gel is compressed and the water is squeezed out. Therefore, the authors proposed that the reacted leaf extract (i.e., the black layer) acts as a lubricant  preventing the contact between wheel and rail.  

Using tribological tests, the authors found that tannins reduce friction in pure sliding as well as in rolling sliding. Regarding mechanism of reduction in friction the authors proposed dynamic lubrication as the most probable mechanism.

It is important to mention that due to the open system nature of wheel-rail contact, the mechanism  proposed in this work is not the only one involved in reducing friction, as highlighted by authors. In addition, the authors also mentioned that the chemicals other than tannins might also be responsible for reduction in friction. Therefore, more work is needed to get a full overview of the composition of black layer and the mechanism involved in friction reduction.

To read the original article from the authors click here.   

Take home for me

I use train daily in Norway for commuting between my office and home. Every now and then the train is delayed (irrespective of the season). Most of the times the announcement at the train station mentions that the delay was due to signal problem. Now after knowing this, I am thinking that probably they don’t want to tell us that we didn’t clean the track well! Next time I will ask the train staff whether the signal was really faulty or there were leaves on the track?


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