I found the sweep profile does not fit to the rail. You can see from the enclosed screenshot, the blue surface is a bit off to the yellow rail curve.
Also it is possible to have option to make the end of the sweep flat?
Can you post a .3dm file with the geometry? You can use the vertical arrow above where you type your post.
Here is an example of the problem, you can see the the sweep surfaces are not fully match the rail. One side is longer, other side is shorter.sweep 1 rail problem.3dm (50.0 KB)
Hello - I see that, thanks for the example. You can make it work correctly by using the ‘subcrv’ selection for the rail - that is, when prompted for the rail, type ‘subcrv’ and Enter and then, using End osnap, designate the part of the rail to use - from the section curve to the end.
I’ll put this on the pile for the developer to look at.
-Pascal
Hello. I am following the Modeling Complex Transitions tutorial.
https://vimeo.com/109184032
I found the sweep 1 rail issue does not work as expected. Rhino version 6.15.19164.21011
Modeling Complex Transitions 004.3dm (1.2 MB) 
No reply to this bug?
Hello - I am not sure why you’d use Sweep1 here - Sweep2 (two rails) seems like the obvious tool - have you tried that?
If your question is about the apparently slightly short sweep - look carefully at the starting end of the curve - it is also short of the shape curve that you are sweeping.
Maybe this is what you’re shooting for:
Modeling Complex Transitions 004_PG.3dm (156.8 KB)
Simplest would be a FiiletSrf at .4 radius to fill the gap.
-Pascal
Hello.
I found the rail is short at the starting end. Thanks for point out that.
Fred