Slippery rail

Slippery rail or low railhead adhesion, is a condition of railways (railroads) where contamination of the railhead causes trains to experience less adhesion (grip). This can lead to wheelslip when the train is taking power, and wheelslide when the train is braking. The most common cause of contamination is 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".

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 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. Although the effect is only short term, its unpredictability can cause a significant incident to occur. A morning dew can have the same effect.

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.

...
Wikipedia