Is it possible for scotrail to put the inter7city HST's on stranraer, kilmarnock and carlisle services? They could open up more stranraer services and don't have the door problem of the class 158/170, obviously they might be too long for platforms but shorter sets could be ran or they could extend the platforms
Technically possible? Yes, of course. Mk3 coaches have been down there many times, and I'm sure there will have been some instances where HSTs did the journey, likely as part of engineering work diversions.
The real question is whether their use is justified, especially considering the fact that ScotRail would have to procure more sets, get them modified and then train traincrew on the new traction.
Overall it would be a very expensive endeavour which I couldn't see happening in a million years, seeing as ScotRail are already under pressure from the drivers' union ASLEF to announce their plans for withdrawing HSTs (in view of the Carmont accident).
I think the best you can hope for is that whatever replaces the 156s is a bit better. But I wouldn't hold out high hopes. In fact I'd say there is a greater chance of the line to Stranraer being
closed than of it seeing HSTs or anything in that vein.