A new train braking technology could alleviate the disruption to rail services caused every autumn by leaves on the line.
This is the claim of CoCatalyst Ltd whose new Water-Trak system has undergone successful trials on a specially prepared slippery rail.
According to Network Rail, 10 million trees shed thousands of tonnes of leaves onto railway tracks every autumn. When trains pass over leaves, the heat and weight of the trains bake them into a thin, slippery layer on the rail.
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When the leaf-based layer is lightly wetted, the resulting poor wheel-rail friction leads to greatly increased braking distances and reduces starting traction.
According to John Cooke, director of Farnborough, Hants-based CoCatalyst Ltd, low adhesion occurs at other times of the year in the presence of rail head contamination – such as iron oxides – and light rain or dew.
“The common thread running through all these examples is the presence of a contaminant and a certain critical amount of water,” he explained via email. “While a dry rail and a fully wetted rail give acceptable levels of friction, a damp or lightly wetted rail is associated with poor adhesion.”
Research into the role of iron oxides on the rail head has revealed insights that may apply to leaf contamination, Cooke added. The research describes three regimes of frictional behaviour, depending on the proportion of water and contaminant (iron oxides) present on the rail head.
He explained that when the solids fraction is lower than around 85%, the contaminant can flow relatively easily away from the contact point, providing good levels of friction between the wheel and the rail.
“When the solid content increases from 0.85 to about 0.97, the contaminant/water mixture is increasingly able to separate the surfaces and carry normal load. Because the mixture still has a limited shear strength it is unable to transmit tangential stresses,” he said. “This results in a significant reduction of the adhesion coefficient. As the solid fraction increases further the shear strength of the contaminant/water mixture increases rapidly, resulting in an increase of the adhesion coefficient and a return to safe braking.”
Using this knowledge, the Water-trak system delivers water to the wheel-rail interface when low adhesion conditions are detected.
“Given the low amount of water and contaminant present, the volume of water needed is minimal – less than 5ml per metre of rail,” said Cooke.
The patented system delivers water in front of the leading axle of the train and consists of a water delivery unit and control unit. Water delivery is triggered when the train’s Wheel Slide Protection (WSP) system responds to low levels of track adhesion.
“The current system pressurises the water to 7 bar and contains approximately 200l – sufficient for at least two weeks operation in Autumn,” said Cooke. “The system has been designed to deliver water within one second of receiving a trigger signal.”
Trials of Water-Trak were undertaken with a hydrogen-powered HydroFLEX train and were supported by rail leasing company Porterbrook and Birmingham University. Testing marked the end of a project funded by the Department for Transport through Innovate UK.