https://drive.google.com/file/d/0Bx5Gjm ... lKWk0/view
I only remember that I had a lot of trouble figuring out a good way to arrange my rails at that point. And then I suddenly noticed this blinking. No mods, Absolutely vanilla. In case you need the save file (I think it won't help because there is no replay saved) I will upload it( via google drive).
The tooltip on the rails on both sides of that signal is indicating that they are belonging to the same block.
EDIT: Possibly Minor Issue because replacing the signal fixes it.
EDIT 2: Here is a save with replay for this:
https://drive.google.com/file/d/0Bx5Gjm ... hoUnc/view
I used the creative mode command that I have created:
viewtopic.php?f=8&t=26854
(This link must work because I have tested it with my friend)