High Adhesion

The adhesion railway which uses adhesion traction is the most common type of railway, where trains are moved by driving some or all of the wheels of the locomotive or railcar. Rail adhesion relies on the friction between a steel wheel and a steel rail, or a rubber-tired wheel and steel rail as in the Montreal Metro, for example. The term may be used to distinguish conventional railways from other types such as funiculars and cog railways.

This article focuses on what happens as a result of friction between the wheels and rails in what is known as the wheel-rail interface or contact patch. There are the good, the traction and braking forces and the sideways forces which contribute to stable running on straight track and curves. There are the bad, which incur large costs for fuel to overcome trailing resistance, for maintenance that is needed to address fatigue (material) damage, wear on rail heads and wheel rims, and rail creep from traction and braking forces. Costly derailments from wheel climb, for example, are caused by friction between a wheel and the rail.

The wheel/rail interface is a specialist subject with continual research being done to more effectively manage the detrimental aspects of friction and reduce costs.

Traction or friction is reduced when the top of the rail is wet or frosty or contaminated with grease, oil or decomposing leaves which compact into a hard slippery lignin coating. Leaf contamination can be removed by applying "Sandite" (a gel-sand mix) from maintenance trains, using scrubbers and water jets, and can be reduced with long-term management of railside vegetation. Locomotives and streetcars/trams use sand to improve traction when driving wheels start to slip.

Adhesion is caused by friction, with maximum tangential force produced by a driving wheel before slipping given by:

Usually the force needed to start sliding is greater than that needed to continue sliding. The former is concerned with static friction (also known as "stiction") or "limiting friction", whilst the latter is dynamic friction, also called "sliding friction".

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Wikipedia