I think most of the rail systems in at least central Europe are high-frequency rail systems. It's standard that you have 60 minutes halfway or 30 minutes halfway on most of the important intercity stretches.
What is the difference? I would say that the important difference is from 160 kilometres onwards. Above 160 kilometres, you need continuous train supervision. Before that, you have a discrete one that's a little cheaper. The infrastructure is not very complex. Above 160 kilometres, it becomes much more complex. It increases exponentially when increasing the speed.
It's not the technical and not the safety system. Again, it's more about building all the civils—tunnels, bridges and that stuff. If you're building for 350 to 400 kilometres per hour, then it really depends on the topography.