The current range is based on the concepts of ‘swing link’ suspension. This is how prototype coaches are mounted on their bogies. It means that the body sways about an axis near the roof, instead of rocking on the top of the bogies. Even on dead flat track there is a need for flexibility in the bogie since the wheels are coned, and any sideways movement will cause one wheel to ride up, and the other down. This means that the liklihood of a derailment is increased for a solid chassis – even on level track. Not only should the bogies have this flexibility, but it is required between the bogies as well. However to achieve this it is important that the bogie pivots permit rotation about the vertical axis but only about this axis.