Well everybody seems to be going into orgasm about these trains but lets see what the refurb is like and how well they work in practice, when I travel on an HST these yes they are more comfortable than most newer stock in many ways but I see a clapped old train that's past its use by date whatever the refurbishment inside, then there is the re engineering of the doors, re engineering of old trains seems to be having mixed results of late. I guess the fact that there are 2 power cars should help and the engines are only mid life but I doubt they will last 20 years as some people suggest and the next franchise will more than likely need to look for a replacement which going to be a very heavy loading with the majority of Scotrail's Sprinters coming up for renewal as well.
As an aside I have always found Scotrail 170's reasonably comfortable but just lacking in capacity in some cases while the benefit of replacing 15+ year old trains with 40+ year old ones remains to be seen.
The Brief for Franchise bidders was to provide stock on the Intercity routes that had:
Better acceleration;
No direct access from outside to the saloon without secondary doors;
More comfortable interior;
It had to be diesel and it had to last until the end of the Franchise.
I think most of us (including Transport Scotland) expected bidders to propose Voyagers or 180s or similar. TS specifically put some clauses in about sorting out toilet smell problems which seemed to suggest Voyagers were on the cards.
So the HST plan was a bit leftfield but actually it fulfills the brief. They don't need to last 20 years (though it would be good if they could squeeze more than the minimum 7 years out of them.
If they are reliable, fast and comfortable then they'll have done the job.
So long as the rolling programme of electrification continues as it has been then sprinter replacement is dealt with via new EMUs. East Kilbride, Kilmarnock, Maryhill and Barassie electrification and you could virtually be rid of the 156s by the end of the Franchise period in 2025.
The challenge for HSTs is whether they can last long enough to allow electrification to have reached northwards so they can be replaced by EMUs in 2030 or so. It seems to me unlikely that Inverness will be wired before HSTs are obsolete so some sort of Bi-Mode replacement may be needed for at least some of the HSTs.
The only pure DMU stock Scotrail will be buying in future will be the scenic stock requirement for the next Franchise from 2030 or so.