|Photo by author|
It’s that time of year again. The colors of the trees are beautiful and vivid oranges, reds, and purples.
In the UK, delays due to leaves are so disruptive that the colloquial phrase of ‘leaves on the line’ has emerged, often jokingly referred to as a fictional excuse for delays. But the innocuous leaf is in fact no laughing matter.
Here’s the scenario. Leaves fall to the ground and some inevitably land on the train tracks. A train runs over the leaves and compresses them to the tracks where they stick fast due to leaf oils. More leaves fall, or get blown onto the track in the wake of the train, and the process of leaf compression and build up continues. The residue black slime that results is remarkably resilient and rainy weather only adds to the problem.
The surface becomes so slick that trains have to accelerate and decelerate much more slowly than normal in order to avoid slipping, which could cause catastrophic accidents. Many trains, if they detect some acceleration due to ‘spinning’ wheels, can automatically apply breaks which lock the wheels, only making the problem worse. A locked wheel sliding along a track will wear and deform along a particular side, creating a ‘flat’. Damage repair can run into the tens of millions for a single season, along with the frustration of delays.
|Tracks of the Canadian Pacific Railway. Credit: Adapted from Mark Stevens|
Commuter trains, with their frequent stops, are affected by this problem much more than freight trains. Often slower travel times are built into autumn timetables, much to the consternation of commuters.
So what’s happening here? Time for a bit of mid-week physics!
Trains rely on maintaining a rolling, not sliding, movement along the tracks, in order to avoid uneven wear on the wheels. To prevent the wheels from sliding, static friction between the steel train wheel and the steel track is needed. Normally as the train moves forward, the point at which the wheel touches the track at any given moment is stationary and not slipping, balanced exactly by the static friction force and the force applied to the wheel.
The strength of friction between any two surfaces is characterized by the coefficient of friction, μ, and this is different depending on whether that friction is static or kinetic (sliding). Some surfaces are ‘stickier’ than others and have higher coefficients of friction.
If a train driver wants to avoid ‘spinning’ the wheels, then she or he must not accelerate or decelerate too quickly. The maximum force a rolling wheel can withstand before sliding at the base is given by this equation:
|Silverliner V. Credit: John Corbett via Wikimedia Commons|
|Photo by author|
|A Network Rail (UK) ‘leaf buster’ train clearing the tracks from leaves.
Credit: Geof Sheppard via Wikimedia Commons
This seasonal problem is not going away anytime soon and rail companies have tried a number of different techniques to combat the foliage problem. Of course, trimming back the actual source of the leaves along the track is an obvious solution. In the days of steam-powered trains, this was done routinely by large teams of men since grass and leaves could easily catch fire from stray sparks and coals. Today the fire risk from diesel and electric engines is minimal and the large amount of manual labor required would be hugely expensive.
Alternative methods of clearing the track tend to be more temporary. Some companies employ ‘leaf buster’ trains which blast the tracks with high-power jets of water, sometimes in addition to ‘sandite’ trains which apply a sandy paste to the tracks to increase friction. There are also passenger trains that automatically release sand when they detect wheel slippage. Even intense laser beams have been shown to effectively disintegrate and blow away the leaves, but at a high cost.
All this from a simple autumnal leaf. Next time your train is delayed by ‘leaves on the line’ you’ll have much to ponder.
By Tamela Maciel, also known as “pendulum”