Rail commuters would possibly snort, however slippery rails aren’t any joke.
It’s formally autumn within the Northern Hemisphere, which for many individuals means a return to darker evenings, cozy knitwear, scorching chocolate, and pumpkin-spiced all the pieces.* However for individuals who handle railway networks, every autumn brings with it a problem: Leaves on the road.
Although mocked by passengers and seen as an excuse, relatively than a justification, for practice delays, leaves falling onto the monitor actually are problematic. The reason being that they modify the interplay between the practice’s metal wheels and the metal rail it runs on.
Trendy trains are extremely environment friendly at shifting freight (and folks), partly as a result of the truth that they expertise so little rolling resistance. Rolling resistance is a pressure that opposes movement, which is primarily brought on by the deformation a wheel experiences below the load of no matter car it’s on. If that wheel has a rubber tyre, it deforms quite a bit, driving up the contact between the wheel and highway, and resulting in a excessive rolling resistance. Cyclists would possibly really feel the impact of this of their drained legs after a trip, particularly if their tyres weren’t totally inflated. Automobile and truck house owners see the result of their gas invoice – the US Division of Power say that 4 – 7 % of the gas in a gasoline (petrol) automobile goes in direction of overcoming rolling resistance. These percentages are even increased for vehicles. In distinction, a practice’s metal wheels deform little or no, which implies that solely a small patch of the wheel – an space in regards to the measurement of a 5 pence piece or a dime – truly makes contact with the rail. This retains rolling resistance low.
It additionally implies that trains function with far much less grip than automobiles with rubber tyres. The coefficient of friction – which on this occasion will be thought of a measure of grip – between a brand new tyre and a dry highway will be as excessive as 0.9.** Between a metal wheel and metal rail it’s extra like 0.5, although in apply, it’s typically a lot decrease than that. However so long as the coefficient of friction stays above 0.17 (Desk 1 right here), a practice can normally get the traction it wants.
And that is the place the leaves are available in.
Leaves are principally water, however additionally they comprise a variety of different substances, together with cellulose, pectin and fatty acids. Once they fall onto a railway line – or are blown there by the turbulence of passing trains, they’ll persist with it. Every time a practice runs over the leaves, they get compressed, releasing these substances. Over time, this leaf mulch transforms right into a darkish, ultra-slippery coating that may be exhausting to take away. [The exact mechanism by which this happens is still somewhat shrouded in mystery – it’s certainly possible that heat generated by the friction between the materials contributes to the formation of this layer].
This leaf mulch acts as a lubricant – it decreases the helpful contact between the wheel and rail. Some have estimated that the coefficient of friction between a wheel and a rail lined in damp leaf mulch will be as little as 0.015 – ten occasions decrease than a practice wants.
This lack of traction makes it tougher for the practice to speed up and attain its cruising velocity, which implies that it takes longer to journey between stations. It additionally makes braking much less efficient, growing the stopping distance by an element of two to 3. So to keep away from overrunning a sign or a station, drivers have to use the brakes a lot sooner than they might on a dry monitor. All in all, a slippery rail results in a a lot slower journey. Higher that than a harmful incident.
So how do railway managers resolve this?
The usual method is to dispatch a fleet of devoted automobiles – known as railhead therapy trains (RHTTs) – to wash and keep the rails. The cleansing is completed through high-pressure jets of water that blast the leaf layer away. The RHTTs may coat sections of the rail in a gooey gel loaded with sand and tiny particles of metal and iron, to extend the friction between the rail and wheel. Between October and December of this yr within the UK, Community Rail RHTTs will deal with 1.04 million miles (1.67 million km) of monitor.
Through the years plenty of different applied sciences have been examined, with some occurring to be adopted by rail firms. They embrace water stuffed with ultrasonically-activated bubbles, and a high-temperature plasma made with nitrogen extracted from the air.
Some even use lasers, just like the Lengthy Island Rail Street in New York, which describes itself as “the busiest commuter railroad in North America.” Amongst its fleet are two Dutch-invented ‘Laser Trains’, which every home – as you would possibly count on – a devoted system for producing laser gentle. The enterprise finish of the laser practice is the unit mounted near the wheels, and it contains of a pair of infrared lasers. When the practice comes throughout a bit of rail coated within the slippery leaf layer, extremely centered pulses of laser gentle are directed onto the rail. Its wavelength is such that it’s mirrored from the rail however absorbed by the leaves. This heats them up so quickly that they vaporize, with out damaging the rail. The present most velocity that the laser practice can journey is, considerably mockingly, relatively gradual – simply 25 mph (40 km/h). However based on their web site, the corporate is at present constructing a standalone wagon that may be hitched onto any practice. This, they are saying, can function at speeds of 60 mph (near 97 km/h).
Simply a few weeks in the past, researchers on the College of Sheffield introduced their invention – a slender, high-pressure stream of air loaded with dry ice pellets (strong carbon dioxide). When sprayed onto the rail, this stream freezes the leaf layer. And because the solids quickly rework into CO2 fuel – by sublimation – it expands, pushing the now brittle leaves away from the rail floor. The system has been trialed over a number of years – initially on-board current RHTTs and now, on passenger trains working on the Northern rail community. Northern’s Seasonal Enchancment Supervisor, Rob Cummings is quoted as saying that leaves on the road are “one of many greatest dangers to our efficiency throughout October and November,” so the community is “very excited to check this new expertise”.
So whereas there’s no simple resolution to the issue of leaf-covered rails, relaxation assured, there are many individuals working exhausting to attenuate their affect.
* Down right here within the Southern Hemisphere, it’s springtime. I’m unsure I’ll ever get used to the upending of the seasons 🙂 #IrishinNZ
** I speak in regards to the coefficient of friction quite a bit in my new ebook, Sticky: The Secret Science of Surfaces. It’ll be printed on 11 November (Europe), 1 Feb (North America), 1 March (remainder of the world)