With the principal exception of thermal damage, most forms of surface damage of rails are caused by normal and tangential stresses at the wheel/rail contact. These stresses result from loading on the rails, which in turn result from the way that a vehicle behaves on the track. Most damage is worse in curves, where tangential loads tend to be higher than in straight track, because these are required in order to steer the bogie (and vehicle) through the curve. Different types of damage tend to occur on high and low rails because of both the different forces acting on them and the different contact conditions. To give a holistic understanding of some fundamental reasons for surface damage and why particular treatments work, this section contains the following:
- Calculations of the tangential forces resulting from a combination of curving and applied traction.
- A description of so-called “shakedown”, which provides a relatively simple method of determining whether a wheel or rail can carry applied loads.