In the video, the rail can be removed (highlighted yellow) until you reach a certain point, but are still very close to it. This occurs in the same manner with the other curved rail on the right side. Maybe I didn't ever notice before, or maybe it started after the fix for this bug, but I'm not sure.
https://youtu.be/zx_Mt7wnD74
[0.15.11] Can't remove rail
Re: [0.15.11] Can't remove rail
Thanks for the nice video showing the problem 
It's now fixed for the next version of 0.15.
It's now fixed for the next version of 0.15.If you want to get ahold of me I'm almost always on Discord.