Rail Adhesion

Rail Adhesion

The term adhesion railway or adhesion traction describes the most common type of railway, where power is applied by driving some or all of the wheels of the locomotive. Thus, it relies on the friction between a steel wheel and a steel rail. Note that steam locomotives of old were driven only by 'drivers' that were connected to the 'side rods' of the locomotive. These were the large wheels. On diesels, traction electric motors (DC or AC) drive all or some of the axles. Both steam locomotives of old and many modern diesels have sand tubes that deposit quantities of dried sand directly to the rail heads directly in front of most/all axles. When an engine slips, particularly when starting a heavy train, sand applied at the front of the driving wheels greatly aids in tractive effort causing the train to 'lift', or to commence the motion intended by the engine driver.

The term is particularly used when discussing conventional railways to distinguish from other forms of traction such as funicular or cog railway For example, the Bernese Oberland Railway "is a mixed rack and adhesion 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. On an adhesion railway, most locomotives will have a sand containment vessel. Properly dried sand can be dropped on to the rail to improve traction under slippery conditions. The sand is most often applied using compressed air.

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.