Slippery rail, or low railhead adhesion, is a condition of railways (railroads) where contamination of the railhead reduces the traction between the wheel and the rail. This can lead to wheelslip when the train is taking power, and wheelslide when the train is braking. One common cause of contamination is fallen leaves that adhere to the railhead (top surface) 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".
Low adhesion caused by weather Railhead contamination caused by weather conditions can occur at any time of year. The leaf fall season causes the most disruption to rail operations. In heavily deciduous forested areas like the American Mid-Atlantic states, New England, many parts of Europe including the UK, and Southern Ontario, Canada, the problem can arise. Where the leaves fall onto a railway route, some collect on the railhead and are then heavily compressed by trains 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. The build-up of this material is incremental, and it is hard enough not to be quickly worn away by the ordinary passage of trains. Winter can provide problems of low adhesion when snow and ice are deposited on running lines. Just as with road vehicles, black ice can cause trains to encounter difficulty when starting away, or can initiate wheel slide during braking. Even summer can have its problems. A light rain shower following a long period of dry weather can sometimes cause similar low adhesion conditions to those of leaf fall contamination. As the water dries it mixes with oxide debris and creates a paste that separates the wheel and rail reducing adhesion. Although the effect is only short term, its unpredictability can cause a significant incident to occur. A morning dew can have the same effect.
Disc brakes add to the problem Before about 1960, most railway vehicles used brake shoes to stop the train by applying pressure on the wheel treads. Since then, disc brakes have increasingly been used, which means that cleaning the compressed leaf material from the wheel tread by abrasion no longer occurs.
Lack of lineside maintenance A report by England's Commissioners of Railways of May 1851 noted that an accident was caused when a small locomotive hauling a heavy train was unable to find purchase on the rail because by the dirty state of the track ballast. It was the duty of the fireman when necessary to dismount from the locomotive and gather track ballast to throw under the driving wheels to maintain grip, but in this particular instance dirty ballast, containing a proportion of earth, failed to achieve the required outcome. 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 increases the supply of leaves thereby exacerbating the problem.
Low adhesion caused by crushed insects There are many substances which can cause low adhesion when they are deposited on the railhead. In Victoria, Australia, train wheels crushing plagues of introduced Portuguese millipedes which were crossing the tracks, caused passenger rail operator V/Line to be penalised more than $700,000 for cancellations and poor punctuality in one quarter of 2001. In 2009, railway tracks at Tallarook in central Victoria were also affected by a Portuguese millipede plague, causing several trains to be cancelled. The crushing of Portuguese millipedes is suspected to have caused a crash between two trains at Clarkson near Perth, Western Australia, in September 2013. Slippery rails caused by caterpillars were reported in Queensland in 1938. Crushed locusts affected train operations on the Otavi Mining and Railway Company in South-West Africa (modern Namibia) in 1924.
Effects
The loss of friction between wheels and rail results in loss of tractive force: the wheels begin to spin, and in some instances the train is unable to move. In braking, substantial loss of friction results in reduced braking force. Braking distances are considerably longer, and in extreme cases the wheels may even lock up, causing the train to slide. Modern locomotives and multiple units are equipped with Wheel slide protection to counter slippery rail conditions. Locked wheels can self-grind flat spots 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. This causes the wheels to go out of profile (known colloquially as 'wheel flats'), which subsequently leads to severe vibration and the need for the wheels to be re-profiled or re-tyred at great expense. In extreme cases, the build-up of leaf material can electrically insulate the wheels from the rails, resulting in a failure of signalling equipment to detect the presence of the train. Where the problem is severe, Track Circuit Actuators fitted to trains can help alleviate the problem. In the United Kingdom, it was estimated that the poor adhesion problems cost the rail industry GBP 355 million (US$449 million) a year.
Mitigation measures
Railhead treatment
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