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Leaves on the line are so dangerous because they make rails slick, meaning trains struggle to speed up and slow down as easily as they ordinarily would. It can also wreak havoc with signalling by blocking the mechanisms that monitor when trains enter various sections of track. A train is delayed due to low rail adhesion once every eight minutes, about half of which caused by leaves on the line according to Network Rail. Trains stood still for almost 2,500 hours last year due to low rail adhesion.
The issue is “way more complex I think than anybody would ever realise,” says Roger Lewis, professor of mechanical engineering at the University of Sheffield, with the network often busy with peak-time trains relying on minute-to-minute timetabling.
Network Rail hopes lasers might be one solution. It has awarded a £700,000 contract to Laser Tribology BV to test whether lasers can replace the high-pressure water jets currently used to clean the railway. The two-phase trial will start away from operational tracks before moving to a live railway environment, with a target cleaning speed of 60mph.
The principle behind the technology is that the lasers vaporise the leaves on the line without damaging the steel track beneath, which some existing techniques do. Laser Tribology’s LaserTrain has already been tested in New York, where its developers reported a 90% reduction in slipping and sliding for 12 hours after treatment during trials in 2019.
The laser-based solution is just one approach to the problem. Lewis’s own method, trialled in collaboration with the Northern train operator, is to use dry ice to remove leaves from the line.
All approaches are attempting to replace the current solutions, which are far from perfect. One blasts water at high pressure to dislodge the plant matter, then applies gel to improve friction (which can be harmed by the treatment), while another uses sandblasting.
Lewis is somewhat wary of the ability of a laser-powered solution compared to simpler alternatives. “I always think, particularly with things that go on trains, the simpler they are the better,” he says. “The more complex something is, the harder it is to maintain and operate.” He acknowledges that his own dry ice solution requires regular top-ups to operate.
Having so many different projects trying to address the issue ought to be a blessing, but an abundance of choice can also be seen as a curse. “The problem is that the industry doesn't know what they want to use, so we'll all come along with our own individual ideas that work in certain ways,” Lewis says.
The technical challenge is only half the battle. Getting a leaf-clearing technology to the track section in question can be tricky, particularly when the network is busy. Damp, shaded sections beneath trees are particularly susceptible to low rail adhesion, and leaf contamination can begin building up again less than an hour after a treatment train has passed.
As well as speed, there is a key question around responsibility. Until the advent of Great British Railways, infrastructure and trains have different managers. Lewis calls that “an interface problem”, requiring cooperation between distinct bodies.
All of this means that there is no let-up yet from the much maligned leaves on the line. “Lots of people are working on new technology, but there’s no clear path to them getting their technology implemented,” Lewis says.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.