The Railway gets its feed from DNO “grid sites”. We star feed off these, with strategic gaps on our HV network to separate grid sites. This stops us running them in parallel unless we are doing switching when we have to check for synchronicity and sometimes get permission to close our gap. We then move our gap to another HV breaker on our network keeping parallel time to a minimum. The Grid sites sync is important as if we get it wrong huge currents can flow causing all manner of tripping/outages and interviews without tea and biscuits.
Sitting at each of our subs stations on our HV network (33kv/22kv/11kv) are auxiliary transformers that supply signalling equipment, normally one on each side of the sub station HV feeders and some sites also have an area board feed as well as an emergency back up.
The problem occurs when we loose the grid site and all the signalling kit tries to switch to the local DNO and if that’s fed from the same grid site…….
The other problem we had was that the loss of the OHL 25Kv upset some off the new trains and a tech with a laptop had to go to each one to reset them! Quite why this caused the above issue I don’t know
( I work in a third rail D.C. control , which has both an HV AC network and a D.C. network)