That's a pretty neat thing you're doing. This issue should be fixable on Unity's side. What is the material you're using?
For reference, I'm testing on Unity with an "Unlit/Transparent Cutout" material and the sorting on the guardrails work correctly.
![](http://i.imgur.com/0NqEh1v.png)
![](http://i.imgur.com/2QDmVvj.png)