OK. This is the rail category, so I'll go out on a limb and assume you are speaking of railroad 'guard rails'.
These are used to help keep equipment on a structure, even though a set of wheels may have derailed. They are usually found over bridges, trestles and inside of tunnels. They are laid between and inside the regular rails that the equipment rides on.
They are tapered together at each end, forming a point, so any approaching derailed equipment will ride between them and the outside rails, keeping the equipment from falling off an elevated structure, or from creating a huge mess inside of tunnels.
At one time they were common place, but now are usually seen only where there is a high degree of probability that a derailment will cause a large amount of destruction or pollution in the event of spilled haz-mat.
Or were you talking about hi-way guard rails? Guide or guard, it makes no difference. The question I would ask is, "Why do they call them 'rails'?" It's not a fence and nothing is supposed to ride on it. I say we get rid of the confusion and rename it as a 'CRS' (Composite Roadside Structure).
But noooooooo. Now I'll be thinking of this on the road tomorrow, lose my concentration and wind up on the CRS. I hope you're happy........