GEML with a class 90 and mark 3's and one EMU, maybe a 315 might not be ideal but would be a good start. There would be scope for DLC EMU packs. TSW is a world away from MSTS and even TSC in many cases, expecting to get a full suite of traction for a route like the GEML is a pie in the sky dream, if it were to happen it would be a long term project and would be much more expensive. I really don't think it is logical to compare TSW with either of those two. For the record I loved the MT products and would Scottish Capitals or London South East to be recreated as they were in MSTS, sadly that is highly unlikely to happen.
Would it not be better to do a 315 (modded 314) and either a 321 or 360, with the 90+mk3s as a separate DLC?