The planing radius is the radius of the ironwork from the physical switch toe. It starts at a distance "T" from the Origin of the switch radius at the switch toe. It has a flatter radius than the switch radius for a certain planing length. This planing radius is not tangential to the through route of the turnout. In real life, the switch cannot be tangential truly due to widths of rail and limitations of ironwork, so planing must occur. I would guess the fact that it isn't tangential is the reason why it is not included in the true geometry type turnout.
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