After being disappointed with a lot of free spline deformation tools, I bought and have been pretty satisfied with a Unity asset called Spline Architect to basically do what you’ve described. The workflow is:
- Make the track spline
- Copy the track meshes along it
- Play, tweak the shape, repeat
- Add scenery
It’s nice because it’s a non-destructive workflow and I can alter the spline and just have the track change to follow it. I don’t have a level editor built into the game yet though, it’s all just in the Unity editor itself. I will probably record a devlog of an end-to-end process of developing a track at some point to show what that’s like and my thought processes while designing it.