My wife has been a member of this group for a while, I occasionally read her posts, but seeing this thread, I have now joined in my "own right".
I retired in June after 43 years of being a railway signalling engineer with the last 35 years specialising in safety engineering.
It will be highly unusual for the signalling system to be at fault (unless caused by the derailment of the first train, which is a circumstance nobody could have foreseen). There are many possibilities and I won't set any hares running by mentioning them here. What I will say is that I can cite many railway accidents as a result of a train driver being give the wrong token and travelling down a single line, where permission has actually been given for a train travelling the opposite way.
We actually have one of the safety rail system in the world, until Stonehaven last year there had not been a passenger fatality on NR for over 10 years.