This really shows how antiquated a lot of our rail network still is. I wouldn't have believed this were still possible. We've had safeguards to prevent this kind of issue since the age of steam.
I'm going to hazard a wild guess that privatisation and tory cuts are the ultimate cause of this.
Would that be the antiquated new digital system to stop trains? Or the antiquated leaves on the line? Or the antiquated steep incline at the location of the crash? Maybe we'll wait for the RAIB investigation report rather than your wild guess.