H Michal,
I’ve done some quick tests, but I can’t reproduce the described behavior here – the 
Mirror option within the 
Linear 1S1 Generator doesn’t “flip normals” in my setup. Also, by using the 
Mirror Operator inside of the RailClone object you shouldn't face any "flipped normals" like while using the 3ds Max's Mirror tool. Also, the 
Mirror or 
Symmetry modifiers should work just fine. Please find attached a sample scene using the 
2Sided material (also the 
xView - 
Face Orientation tool doesn’t show any issue(s)). In case of any further troubles with it, if possible, please feel free to 
send us your scene (ideally simplified and containing just this problematic object(s)). Ideally, if you'll be able to reproduce it on a minimal scene, which you can send us, it would be great. Thanks in advance.
Best regards,