You can face an issue when creating a regression line from rails, even rails are just basic linestrings. The issue is that it takes too much time and you will get too many points. The curvature diagram is sometime incorrect as well. To avoid such issue:
Reference your rails in the 2D model of your design file
Using Model detailing>Plan by 3D, create your rails LR and RR in the 2D model by selecting the rails from the reference