The analysis shows that dwell time is very important to overall capacity and performance (especially at Old Oak Common and Birmingham Interchange) and a level floor inside the units if key to achieving this.
The appropriate EU rail authorities disagree and note that a level floor within the vehicle is the primary constraint. They have published numerous expert opinions on this fact.
They declare that level boarding is by far the most important factor, and that can be achieved with the existing standards.
Not many people actually move between carriages on modern trains, they board and stay where they boarded, especially on commuter trains (Which is effectively what this is)
The number of additional passengers you can get on double deck unit is actually quite small especially when you lose decent overhead luggage rack space which you then have to compensate for by losing floor space.
How many overhead luggage rack spaces do things like Class 185s have near the doors, for example?
Overhead luggage space is a dying thing on modern British trains, given the short journeys (the average captive journey will be under an hour) the lower deck of a double deck train would likely be fitted out like the vestibules in a Class 185, small amounts of seating and large amounts of standing space.
When you look at Pax / hour with planned infrastructure, the level floor ~ 1200mm wins in capacity terms.
Using somewhat heroic assumptions on the part of HS2, which numerous EU rail authorities have repeatedly attacked.
The space under a 1200mm floor is also needed for all the equipment especially as the desire is for a flexible open passenger space so the interior layout that can be reconfigured and the space used most efficiently (and no power cars).
It is somewhat telling that the only EU rail operator to try double deck high speed trains has ordered nothing but double decks since!
The capacity of an Avelia Horizon is supposedly something on the order of 740 on a 200m unit, as opposed to ~450 or so for a 200m Velaro-D.
The shrinking size of the equipment on board the train as technology improves is a rather beneficial part of this.
To be honest I agree, with EU rail authorities, that HS2 has chosen a specification designed specifically to render all rolling stock for the service entirely bespoke, so that they can de-facto exempt themselves from open access and other provisions.
The adoption of a GC loading gauge is entirely pointless unless a platform height is chosen that will permit double decker trains.
If Old Oak Common is not capable of handling standard TSI trains, then additional platforms should have been added to keep within the specification.
All expectations are that double decks have 30-40% more capacity than single decks, and unless improving OOC and Birmingham International would increase entire project costs by that fraction, which it won't, then this is a bad economic decision.
EDIT:
Does anyone know what is happening though?
I have to assume the decision has been made either way, given that construction has already started?