34.1.2 Wheel/Rail Interface and Axle Loads
Since the adoption of steel rather than iron as the material of choice for rails, the wheel/rail system has
remained virtually unchanged. However, this does not imply that the system is ideal. The wheel/rail
contact area is typically the size of a small coin and, commonly, eight such contacts (i.e., eight wheels)
support a vehicle weighing from 30 t (lightweight passenger coach) to 140 t and more (heavy freight).
The material in and around the contact area is therefore highly stressed. High rates of wear might be
expected for such a contact but, in addition, because the load is applied and removed many times during
the passage of each train, there is the possibility of fatigue of the rail surface. Further details of these
loads and their effects on steel rails and wheels are discussed below. The ideal material, which does not
wear or suffer fatigue and yet is economically viable as a rail or wheel material, has not yet been found.
The axle-load examples given refer to the static loads applied in the contact patch area when a train
is stationary. Dynamic loads (e.g., at rail-joints or in turnouts [points]) are much higher, with vertical
accelerations reaching values of 100 g (1000 m/s
2
). This is a consequence of the high stiffness of the
wheel/rail interface. Well-designed primary suspensions are essential to minimize the impact of these
loads on track life and wheel life.
Heavy freight trains are generally limited to 60 to 90 km/hr, the top speed being determined by
gradients, aerodynamic considerations, and the capability of rolling stock. On high-speed passenger
routes such as the French LGV (
ligne grande vitesse
or high-speed line), axle loads are limited to 15 t, a
constraint necessary because of the much higher dynamic forces at 300 km/hr.