In the process of developing our software, we have been using Autodesk’s Civil 3D to check our results.  When some comparisons showed deviations, we were surprised to find that the errors lay not with our work, but with the venerable Civil 3D.  Lengthy correspondence with Autodesk’s developers and a recent (June 2, 2009) meeting at their office confirmed our conclusions.  Autodesk has been working hard to fix their problems and should be caught up soon.  Meanwhile, if you are doing high-speed rail design, use caution with compound spirals!