Hi Pascal,
I'm afraid you're not going to like it, but my method is mainly manual. I would love to have something automatic, but nothing worked as expected. It depends a lot of the track I guess, in my case this is what gave the best results [using Blender]:
Step 0 - so you can see what you are dealing with:
- into object mode, select solid shading
- select flat shading as well
- for material choose something like in screen - this allows to really see the bumps
Step 1:
- switch to Edit mode
- select all (A)
- press W >> remove all doubles (if the track is from rF, it probably is the case)
Step 2:
- poly selection mode, select the polys that were higher and manually adjust the height until the gap is less visible (using SHIFT it gives higher mouse precision). I know it sound like a tedious job, but it's not that much actually. It doesn't need to be perfectly aligned either, because the next step will take care of the rest.
Step 3:
- in vertex selection mode, select all the verts, except for the contour ones (screen bellow)
- press Space >> vertex smooth
Step 4:
- switch back to Object mode
- set the shading back to smooth
Even if you'd still be able to spot small issues, they will be unnoticeable in AC, or just slightly noticeable which will actually give the track some character, and not make it run smooth like a piece of paper.