In some circumstances Rail Revolve is shifting the start of the surface away from the profile.
In the attached model a circle has been split into four quadrants and each quadrant rail revolved around the long axis. They should all end up at the equivalent quadrants around a finishing circle but only the two quadrants that lag behind the rail do so. The other two are shifted back so that they too lag behind. (And if you try the same thing with a complete circle, it too gets displaced.)
Rail revolve.3dm (132.6 KB)
Is this by intent? I hope not but if so could the documentation make it clear that you have to set the rail alongside the foremost point of the profile?
Incidentally, if you try to move the start of the displaced railed surface back to coincide with the profile it seems that the end of the railed surface position is slightly inaccurate because it doesn’t quite line up with the finishing circle.
Rhino 6.9.18240.23591, 28/08/2018