Rail traction - Force-controlled grip

Friction between two bodies in contact governs all our movements, especially on land and rail, where it is the scene of singular acuity. Steel wheels and rails have a very low coefficient of friction: an unrivalled advantage in terms of energy efficiency for traction, but a delicate counterbalance when it comes to braking effort. This is the only explanation for the layout of the railroad lines built in the 19th century: ramps and the smallest possible gradients shaped the so-called "valley" routes (Table A).

The development of electric traction once again posed the problem of adhesion. Effort, speed, and therefore power, are in no way comparable with steam traction, and could they be applied to the same wheel-rail contact? That's what this dossier sets out to demonstrate.

After recalling the process of friction at wheel-rail contact, we will describe the stages and current achievements in the field of traction by commutator motor, then induction, as well as devices for regulating wheel slip in relation to the rail, in traction and braking. The results achieved are decisive in terms of towed loads for traction, and limiting speeds in terms of stopping distances for braking. The very specific case of the rubber-tyred metro is mainly justified by the need to increase grip, which is the same as for road transport.

Adhesion management, far from being a definitive solution, is the subject of ongoing development. It represents a major argument for the increased performance required if railroads are to retain their advantage as the most energy-efficient form of transport, and one of the driving forces behind sustainable development.