force to make it happen, and the friction force resisting the sliding is designated "L", the lateral force. The wheelset applies a force L outwards to the rails, and the rails apply a force L inwards to the wheels. Note that this is quite independent of "centrifugal force". However at higher speeds the centrifugal force is added to the friction force to make L. The load (vertical force) on the outer wheel is designated V, so that in Diagram 4 the two forces L and V are shown. The steel-to-steel contact has a coefficient of friction that may be as high as 0.5 in dry conditions, so that the lateral force may be up to 0.5 of the vertical wheel load. During this flange contact, the wheel on the high rail is experiencing the lateral force L, towards the outside of the curve. As the wheel rotates, the flange tends to climb up the flange angle. It is held down by the vertical load on the wheel V, so that if L/V exceeds the trigonometrical tangent of the flange contact angle, climbing will take place. The wheel flange will climb to the rail head where there is no lateral resistance in rolling movement, and a flange climbing derailment usually takes place. In Diagram 5 the flange contact angle is quite steep, and flange climbing is unlikely. However, if the rail head is side-worn (side-cut) or the flange is worn, as shown in Diagram 6 the contact angle is much flatter and flange climbing is more likely. Once the wheel flange has completely climbed onto the rail head, there is no lateral restraint, and the wheelset is likely to follow the yaw angle, resulting in the wheel dropping outside the rail. An L/V ratio greater than 0.6 is considered to be hazardous. It is emphasised that this is a much simplified description of the physics; complicating factors are creep, actual wheel and rail profiles, dynamic effects, stiffness of longitudinal restraint at axleboxes, and the lateral component of longitudinal (traction and braking) forces. Rerailing Following a derailment, it is naturally necessary to replace the vehicle on the track. If there is no significant track damage, that may be all that is needed. However, when trains in normal running derail at speed, a considerable length of track may be damaged or destroyed; far worse secondary damage may be caused if a bridge is