Slippery rail is a condition of railways (railroads) caused by fallen moist leaves that lie on and cling to the top surface of the rails of railway tracks. The condition results in significant reduction in friction between train wheels and rails, and in extreme cases can render the track temporarily unusable. In Britain, the situation is colloquially referred to as "leaves on the line".
In heavily deciduous forested areas like the American Mid-Atlantic states, New England, many parts of Europe including the UK, and Southern Ontario, Canada, where there is heavy leaf fall in the autumn, the problem can arise. Where the leaves fall across a railway route, some of the leaves fall on the railheads; and at the passage of the next train, they are heavily compressed into a slippery low-friction coating on the rail and on the wheel treads. If the climate is damp, the wet leaves adhere to the rail very effectively. The draft caused by the passage of the train causes nearby leaves to be caught up in air currents, and more leaves are deposited on the railhead.
Before about 1960, most railway vehicles had brake shoes that applied pressure to the wheel treads. Since then, disc brakes have increasingly been used instead. Brake shoes have some effect in cleaning (by abrasion) the compressed leaf material from the wheel tread, but in other respects[which?] disc brakes are superior.
In the steam locomotive era, trees and other lineside vegetation would be regularly cut back to reduce the risk of their being ignited by sparks from the locomotive. As the railways ceased to use steam traction, this maintenance was allowed to lapse, and the resulting extra growth increased the supply of leaves thereby exacerbating the problem.
The loss of friction between wheels and rail results in loss of tractive force — the train cannot move forward and the wheels merely spin, uselessly. In braking, substantial loss of friction results in the braking becoming ineffective, with the wheels locking. (Modern traction and braking control systems modify these effects and are not elaborated here, but the end result is similar.) Locked wheels can self-grind "flats" on the steel tyres, especially if the wheels are still sliding when arriving at a non-greasy section of rail, e.g. one that has previously been sanded. These flats subsequently cause severe vibration and the wheels must be re-profiled or re-tyred at great expense.
In extreme cases also, the build-up of leaf material can electrically insulate the wheels from the rails, resulting in a dangerous failure of signalling equipment to detect the presence of the train.
In summary, the railway can quickly become inoperable, or incur serious and cumulative delay as trains are unable to maintain normal point-to-point timings.
Railways' mitigation measures generally involve some system to jet or blast the accumulated deposit away, or to coat it with a high-friction material. Blasting is usually carried out with water jets, often in combination with mechanical scrubbing apparatus. The coating method usually involves depositing sand in a paste on to the rail; as the sand may exacerbate the risk of unwanted insulation, the sand mix sometimes contains ferrous particles. The coating is applied from special trains (colloquially referred to as "Sandite" trains after the original proprietary mixture applied) and in some cases locally by hand.
Both of these processes are effective for a limited duration; the jetting method is ineffective as soon as the next leaf falls; the sand deposition method is more durable, although rainfall usually removes the deposited sand quickly.
Train handling is modified in areas and at times when leaf fall is expected; sharp acceleration and braking are avoided. In the UK a special, somewhat less congested, timetable is published and operated at these times.
Removal of deciduous trees at the lineside is a management method to control the problem; however, there is political resistance to this in populous areas.
Slippery rail has created severe disruptions of rail service, particularly in major metropolitan areas such as New York or Boston. In November 2006, it was blamed for roughly one-third of all Metro-North Railroad's Hudson and Harlem lines' passenger cars being taken out of service. During the same period on the Long Island Rail Road, nearly 25% of cars were out of service due to slippery rail.
In the US, Amtrak, the Massachusetts Bay Transportation Authority, southeastern Pennsylvania's SEPTA, Chicago's commuter rail service Metra, and MARC, which serves Baltimore and Washington, D.C., have all reported delays due to slippery rail.
Methods for dealing with slippery rail have included trimming trees, the release of sand on the train wheels for traction, high-pressure water blasting and, most expensively, the use of high-powered laser blasts to clear the rails of leaves.
Metro-North has designed a system dubbed "Waterworld," which is a large flat rail car that blasts the rails with high-pressure water jets as the car moves over it.
New Jersey Transit has used a similar method, which has proven effective. The device it uses is called "Aqua-Track" which, while attached to a moving rail car, sprays water at a pressure of 100,000 psi (690 MPa) on to the part of the rail where the leaves cling. Since this system was introduced in 2002, the delays due to wheelslip have been reduced by over 60%.
SEPTA Regional Rail's method of preventing slippery rail is the Gel Trains. These two trains spray a high-pressure mixture of sandite on the rails; in the fall, the Gel Trains also clean the rails using the high-pressure water jet method ahead of the gel application. These trains consist of a pressure washer and gel dispenser mounted on a converted flatcar, and a tank car which carries water. They are pulled on one end by one of SEPTA's work diesels (or a diesel borrowed from a local shortline such as the West Chester Railroad), and controlled at the other end by a former LIRR "Power Pack" cab unit (one a former ALCO FA, the other a former EMD F7).
The cryptic nature of rail company explanations for slippery rail and related phenomena has led to the phrase "leaves on the line" becoming a standing joke, and, along with variants such as "the wrong type of snow", is seen by members of the public who do not understand the problem as an excuse for poor service.
Particularly problematic local trees include the sycamore, lime, sweet and horse chestnut, ash, and poplar, which regrow or coppice after cutting back, and have large, flat leaves, which stick to the line and cause severe slippery rail. Other types of tree that cause problems are quick-growing, pioneering trees, or those producing a substantial amount of leaves. Poplars are particularly troubling because they tend to shed limbs.
The current term for cutting down or cutting back trees near the lines is "lineside vegetation management".
In 2013 Greater Anglia reported that the compressed leaves had resulted in a "teflon-like substance" which has baffled scientists. Attempts to mitigate this phenomenon with gels have failed to provide enough traction to provide safe passage. 
Slippery rail is also a problem in the Netherlands, addressed by Nederlandse Spoorwegen and ProRail. To prevent wheel lock, on some routes trains are required to brake earlier and accelerate more slowly. Furthermore, some (passenger) trains are fitted with equipment to apply Sandite gel on the tracks.
Similar problems have been attributed to crushed migrating caterpillars in Victoria, Australia in 2012 and crushed locusts on the Otavi Mining and Railway Company in German South-West Africa (modern Namibia) in 1924. Slippery rails caused by caterpillars were reported in Queensland in 1938.
The crushing of millipedes is suspected to have caused a crash between 2 trains at Clarkson near Perth in September 2013
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