Adhesion railway

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.

The "factor of adhesion", being the weight on the driven wheels divided by the theoretical starting tractive effort, was generally designed to have a value of 4 or slightly higher, reflecting a typical wheel–rail friction coefficient of 0.25.

Other factors affecting the likelihood of wheelslip include wheel size, the sensitivity of the regulator and the skill of the driver.

Toppling will occur when the overturning moment due to the side force (centrifugal acceleration) is sufficient to cause the inner wheel to begin to lift off the rail.

In practice, the minimum radius of turn is much greater than this, as contact between the wheel flanges and rail at high speed could cause significant damage to both.

For very high speeds, the minimum adhesion limit again appears appropriate,[clarification needed] implying a radius of turn of about 13 km (8.1 mi).

In practice, curved tracks used for high speed travel are superelevated or canted, so that the minimum radius of curvature is closer to 7 km (4.3 mi).

During the 19th century, it was widely believed that coupling the drive wheels would compromise performance, and this was avoided on engines intended for express passenger service.

The weight of locomotives was restricted by the stress on the rail, and sandboxes were required, even under reasonable adhesion conditions.

The rubbing of a flange on the track dissipates large amounts of energy, mainly as heat but also including noise and, if sustained, would lead to excessive wheel wear.

Some railway systems employ a flat wheel and track profile, relying on cant alone to reduce or eliminate flange contact.

Understanding how the train stays on the track, it becomes evident why Victorian locomotive engineers were averse to coupling wheelsets.

The kinematic description of the motion of tapered treads on the two rails is insufficient to describe hunting well enough to predict the critical speed.

Restraining springs, opposing the yaw motion of the wheelset, and similar restraints on bogies, may be used to raise the critical speed further.

This may appear trivially simple from a superficial glance but it becomes extremely complex when studied to the depth necessary to predict useful results.

Rails and railway wheels are much stiffer than pneumatic tyres and tarmac but the same distortion takes place at the region of contact.

The definition of creep[9] in this context is: In analysing the dynamics of wheelsets and complete rail vehicles, the contact forces can be treated as linearly dependent on the creep [10](Joost Jacques Kalker's linear theory, valid for small creepage) or more advanced theories can be used from frictional contact mechanics.

Provided the radius of turn is sufficiently great (as should be expected for express passenger services), two or three linked wheelsets should not present a problem.

The friction can vary a great deal, but it was known on early railways that sand helped, and it is still used today, even on locomotives with modern traction controls.

Some of the starting requirements were a challenge for steam locomotive designers – "sanding systems that did not work, controls that were inconvenient to operate, lubrication that spewed oil everywhere, drains that wetted the rails, and so on.."[12] Others had to wait for modern electric transmissions on diesel and electric locomotives.

The frictional force on the rails and the amount of wheel slip drops steadily as the train picks up speed.

During the transition from the "all-stick" no-torque to the "all-slip" condition the wheel has had a gradual increase in slip, also known as creep and creepage.

High adhesion locomotives control wheel creep to give maximum effort when starting and pulling a heavy train slowly.

It can cause a "sandfilm", which consists of crushed sand, that is compressed to a film on the track where the wheels make contact.