I can't add it

Yes. The curves have the same point count but the way they are lined up will cause that twisting. Try to line them up:

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hah… i write about it and dont see it myself thanks @wim

in this file your curves don’t meet the rules I posted earlier:

-rail curves have identical structure (one is 11 control points, the other 13)
-end points of cross section is on end point (as @encephalon clearly pointed out)

Easiest way to do this is by making one rail, and copy this over, then modifying the control points

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