Do you mean Intercity 225 tran without Mark 4 DVT's and insted 2 x Class 91 and up to 16 x Mark 4?
If 225's didn't have DVT's, they would have to modify one Mark 4 coach per set to include a luggage van and Train Manager's accommodation- A bit like the HST's. I like the idea of 16 Mark 4's though.
Speaking about lengthening them, would it be possible to increase the length of the 225's to 11 coaches (loosing about 3 sets in the process), and replacing these sets (and maybe adding additional sets) with pendolino's? Could the 91's cope with 11 coaches and how many platforms are long enough?
Maybe the pendolino's could be used for services to Edinburgh to Glasgow with few stops. I suppose if they had the money to invest, they could then install TASS to enable them to run up to 125mph when the regular line speed is lower (I know it will probably never happen, it's just a thought).
Would there be a way around the in cab signalling problem for running at over 125mph? I was reading a document about current and possible future speeds on the ECML that I found a link to on here a few days ago, and it suggested 155mph running was possible between Doncaster and York, and just north of York. The only problem is, ERTMS isn't going to be here for a long time! It would get the 225's (and pendolino's) to run at 140mph though.