What would happen if you replaced the iron belt in the attached image with underground belt? Would it trace the belt, replace part of the copper belt, replace nothing or something completely different?
Also: What happens with items on the replaced belt? are they moved to your inventory or placed on ...

I got the same problem. In my 0.12-save I had two single trains close to each other, separated by a signal. After updating the rail between the two trains is in the same state as described above. This happened in multiple locations.
railProblem.png
The savefile is attached.

Edit: When attaching a ...