Hmm - the whole HST is a unit argument can be applied nicely to the through wired 37s and Mk2s used on Leeds to Carlisle trains a little while ago.
There's a locomotive at both end. Both work at the same time. The locomotives can come off, move around, and work independently of coaches and each other. The locomotives can if need be haul other things. Label the differences if you will. As far as I can tell, there aren't many between a proper through wired 37 and coaches set and a HST set
And HSTs can run in various forms, they don't actually need to have 2 powercars on a train at all, they can run without anything else too.
You can run small HSTs, big HSTs, HSTs with a few coaches, HSTs with no coaches, first class only HSTs, standard only HSTs.
You can mention the corridor connection. Lots of locomotives had corridor connections in the 60s.
The office was merely in place of a cab, and is gone now in any case.
What is the difference between your coupled TNT through wired 37s on a rake of Mk2s then, with a HST?