To say that something has gone terribly wrong would obviously be a drastic understatement, head on collisions are relatively rare in the railways of the western world, when there are crashes they're usually either into the rear of a preceding train or occasionally into another train which is changing lines in front (or of course, derailment due to poor track or train maintenance).
The fact that this was a single line as well only adds to the rarity of such a thing happening, generally speaking the safety systems of European trains are supposed to stop two trains being on the same track at the same time.
So either it was human negligence caused by overriding or ignoring safety systems (the PZB I believe it's called), or a failure of the safety systems that are supposed to stop this.
