Filled towel rails have a certain amount of the filling liquid inside them. This liquid is the main conductor of heat inside the rails. Being heated by an electric element, it circulates inside the curves of the towel radiator, warming its surface and letting it give off the required amount of warmth.
The way this filling liquid circulates in the system provides a more even and adequate dispersion of warmth than in those towel rails, which are equipped with a heating core. Filled towel rails are more secure as any failure in the heating core requires a more profound repair.