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Research project: The Adhesion Rail Riddle - Ensuring Trains Can Brake

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Applying and evaluating novel ultrasonic cleaning to “leaves on the line”

The problem

Each Autumn the UK rail network loses around £50 million because of the ‘leaves on the line’ problem. This phrase is misleading. It should be called ‘Slippery paste that hardens as is really difficult to clean off the railway lines’ problem. The resultant lignin coating is made from a paste of leaves that are ground onto the rail head by the passing train wheels.

Trains rely on traction (adhesion) between the wheels and the railhead to both accelerate and to slow down effectively. Every Autumn, the slippery coating of squished leaves interfere with this adhesion, the result is that the trains cannot brake safely. Trains must therefore travel more slowly, which leads to delays.

Delays costs the network financially, and also incurs loss to the people, business and institutions that rely on the efficient functioning of the rail network. It also causes loss in reputation of the railways and those who rely on them, and infuriates the passengers.

The Autumn leaf fall
The Autumn leaf fall
The dried paste on the track
The dried paste on the track
All trains rely on traction
All trains rely on traction

What is the current fix?

There is no ideal fix. There exist a few rail vehicles equipped with pressure washers, but these washers are so powerful that they cut through the rail tracks if operated whilst stationary.

Grip can be improved with "Sandite" (a gel-sand mix) applied by special sanding trains.

Rail side vegetation management can help, but cutting down our national woodland is not a practical or ethical solution

The pressure washer train
The pressure washer train
Rail Safety and Strategy Board
Rail Safety and Strategy Board

What are we doing to help?

Our research project was funded by RSSB (Rail Safety and Strategy Board). We wanted to develop ways to remove the hard slippery coating using bubbles and ultrasound, using a technology grown from the successful StarStream® technology.


Stage 1: Make the rails dirty

In order to build equipment that can be used by the railways, we need to build and test our solutions in the lab where we can develop and adjust parameters.

That means that we first had to recreate the passage of a train on leaves in the laboratory. To do this we modified a ‘rubbing’ test machine to repeatedly squash leaves on sample of track using loaded ‘non-stick’ rollers, as shown in the film below:

Reciprocating Laboratory rig used to compress the leaves at realistic pressures (700 MPa)
Reciprocating Laboratory rig
Laboratory generated ‘paste’
Laboratory generated ‘paste’

We also took our leaves to the local railway (with help from Siemens and South West Trains) and had a train roll back and forth over the leaves. The film below was recorded to show this process:

Stage 2: Characterizing the leaf contaminant on the rails

The leaf contamination was characterized by area coverage (ImageJ), surface profilometry (both 2D and 3D) and fluorescence microscopy.

Surface profilometry and coating thickness meters allow us to determine the thickness and coverage of the leaf-film. Below is a short film showing us measuring the film thickness at Eastleigh railway works.

Stage 3: How good is ultrasound at cleaning the rails?

We compared how well two current techniques, the ultrasonic cleaning bath and the StarStream® technology cleaned the laboratory generated leaf film. The film below shows that, whilst the ultrasonic cleaning bath could not remove the leaf paste, StarStream® technology was able to clean the sample successfully.

We also took Starstream® along to the railway works to clean off the leaves we had them run over with their train. Below is a video showing how successful it was.

Stage 4: What next?

The success of StarStream® at removing the leaf contaminant shows the promise of new ultrasonic technology. It can certainly clean rails if mounted on a walking-pace trolley. However to be really useful to the rail network, it needs to be adapted to clean on training running at 60 mph. This might be achieved by

  • Adding to regular trains some 50 m of rolling stock equipped with nozzles;
  • Providing a dedicated cleaning train of similar length to that on which the pressure washer system is currently mounted (but which uses less power, water, and is less damaging to track and to the foundation on which the track is mounted, and does not throw up stones which can damage rolling stock, as pressure washers can);
  • Build a dedicated device (an Ultrasonic Rail Mop, URM) for cleaning leaf contaminant off track.

We are currently testing a URM using a Transport Technology Research Innovation Grant (T-TRIG) from the Department of Transport.



We are grateful to the Rail Safety and Standards Board (RSSB) for part-funding this work through their ‘Adhesion Riddle’ program, and the Department of Transport for part-funding this work with a Transport Technology Research Innovation Grant (T-TRIG). We are grateful to Siemens and South West Trains for access to lines and local resources. Also both Eastleigh mainline Railway Works and the Eastleigh Lakeside Steam Railway for providing rail.


Associated research themes

Engineering and the Environment


Related research groups

Acoustics Group
national Centre for Advanced Tribology at Southampton (nCATS)
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