I'm not sure what "head rail" you're referring to, as the upper bunk is open at both ends.
I've been confused by that too as on who has slept on it before. I was 99.9% sure that there was no "head rail".
They must only put them in when there are kids in the room. We've done four DCL cruises with our two kids (Fantasy, Magic), and there has always been a rail at the head of the top bunk. Just like the one on the side (might be shorter, I've never measured, but same design). I *think* there was one at the foot too, but I'm much less certain about that.