As it happens, I was talking to my mate about this yesterday. They seemed to run fine all day on Monday on battery with the diesel running to top up the batteries.
Yes and that's quite impressive. And I presume that the power
was off on Monday, given that the delays were happening.
The issue is the time loss whilst the train is stopped to talk to the signaller. In future, that could be accounted for in an amended schedule. With nothing running to Caerphilly, the trains could run later to Coryton and back.
The delays do not seem to have been very significant. In fact almost every train on Monday seems to have made up the time by the time it left Queen Street anyway.
What is not obvious to me (but might be if I knew more about railway operations) is why on a railway where drivers don't have to be talked through temporary speed restrictions by a signaller it's necessary for a discussion every time they approach the area where the train wants to use the pantograph but can't.
Especially as it's not even as complex as being able to take power in some places but not others.
I'm not sure it's great PR to bring in new trains with a (most deserved) fanfare and then suddenly remove them without warning, bearing in mind that if someone is just using the Coryton line it's not at all obvious that there is engineering work going on and that therefore things would be expected to return to normal next week.
It may be a coincidence but I don't think it's my imagination that Coryton trains have become significantly busier at peak times since the 756s came in.