The train definitely went through a narrow tube-like tunnel with stewards in each car making sure that nobody tried to poke their heads out of any of the windows. I've always called it the 'Rat Hole' but am I wrong?
I don't think you are wrong.
The line colloquially known as the rat hole was indeed the Up Empty Carriage line, which ran from just south of the Primrose Hill slow line tunnel, branching off from the Up Camden Road line, which itself branched off the Up Slow line, reappearing on the down side further south. It was definitely wired - would have been useless otherwise!
As distinct from the Up Engine line, which ran parallel with the Down Departure line (previously the Down Empty Carriage line), but 'wrong road' i.e. right hand running, much nearer to Euston station.
Both the Up Empty Carriage line and the Up Engine line are gone, but the Down Departure line remains in regular use.
One of my first installation jobs as a trainee in the early 1970s was upgrading the Down Empty Carriage line to passenger standards to become the Down Departure line (which included changing various platform departure route indicators to show 'D' instead of 'C').
I remember at the time fancying a walk through the rat hole but that was a definite no no without a possession, due to the lack of refuges.