Slippery rail

A Network Rail "leaf buster" uses a high-pressure water jet to clear fallen leaves in the United Kingdom

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 loss of 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".

Causes

In heavily deciduous forested areas like the American Mid-Atlantic states, New England, many parts of Europe including the United Kingdom, 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 draught 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.

Disc brakes

Before about 1960, most railway vehicles used brake systems in which brake blocks applied pressure to the wheel treads. From that time, disc brakes came into increasing usage. Brake blocks applied to the wheel tread had some effect in cleaning (by abrasion) the compressed leaf material from the wheel tread. It is generally agreed that the advent of disc brakes made the effects of leaf fall worse.

Effects

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-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.

In the United Kingdom, it was estimated that the autumn leaves problem cost the rail industry GBP 60 million (USD 115 million) a year.

Mitigation measures

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.

North America

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 cutting back trees adjacent to rails, 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 quite 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) directly 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 too has a method of preventing slippery rail, the Gel train. This sprays a high-pressure mixture of sandite on the rails.

United Kingdom

In the United Kingdom, a number of rail companies change their timings and publish special "leaf fall timetables".

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. Further traits that cause problems are being quick-growing, pioneering trees, or 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".

Netherlands

Slippery rail is also a problem in the Netherlands, addressed by Nederlandse Spoorwegen.

Other causes

Locusts

The Otavi Mining and Railway Company had a problem with slippery rails caused by crushed locusts.

See also

• Sandite
• Sandbox (railways)