Rail adhesion

Adhesion railway or adhesion traction is the most common type of railway, where power is applied by driving some or all of the wheels of the locomotive. Rail adhesion relies on the friction between a steel wheel and a steel rail. The term is particularly used when discussing conventional railways to distinguish from other forms of traction such as funicular or cog railway.

Traction or friction can be reduced when the rails are greasy, because of rain, oil or decomposing leaves which compact into a hard slippery lignin coating. Measures against reduced adhesion due to leaves include application of "Sandite" (a gel-sand mix) by special sanding trains, scrubbers and water jets, and long-term management of railside vegetation. On an adhesion railway, most locomotives will have a sand containment vessel, to apply sand onto the track; this is called "sanding".

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, referred colloquially to as "stiction", or "limiting friction", whilst the latter is dynamic friction, also called "sliding friction".

For steel on steel, the coefficient of friction can be as high as 0.78, under laboratory conditions, but typically on railways it is between 0.35 and 0.5, whilst under extreme conditions it can fall to as low as 0.05. Thus a 100-tonne locomotive could have a tractive effort of 350 kilonewtons, under the ideal conditions (assuming sufficient force can be produced by the engine), falling to a 50 kilonewtons under the worst conditions.

Steam locomotives suffer particularly badly from adhesion issues because power delivery is pulsed (especially in 2- or most 4-cylinder engines) and, on large locomotives, not all wheels are driven. The "factor of adhesion", being the weight on the driven wheels divided by the theoretical starting tractive effort, was generally designed to be a value of 4 or slightly higher, reflecting a typical wheel-rail friction coefficient of 0.25. A locomotive with a factor of adhesion much lower than 4 would be highly prone to wheelslip, although some 3-cylinder locomotives, such as the SR V Schools class, operated with a factor of adhesion below 4 due to their smoother power delivery. Other steam locomotive design factors significantly affecting traction include wheel size (smaller diameter wheels offer superior traction at the expense of top speed) and the sensitivity of the regulator.

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