Capacity needs to be maximised on all HS2 trains
Then selecting a floor height which cripples double deck solutions might be a mistake.
1115mm is not going to permit double deck trains with a continuous level deck on the GC loading gauge.
If we had more height it would be a different story, but we don't.
You will end up with awkward solutions with huge staircases.
915mm might have been achievable but there simply is not enough height above 1115mm.
But not with level floors through the trains (this also allow interior flexibility and capacity maximisation)
Stadler managed to provide this didn't they?
The traction equipment has to fit somewhere and also maintain a low centre of gravity hence under the floor (as power cars are also a no-no which was the traditional Alstom and Talgo alternative).
It should concentrate on it's capacity and performance specifications, not start defining how the trains should be engineered.
The recent improvements in technology have seriously reduced the disadvantage of the power car solution (ie. the power cars are getting much shorter), it is not particularly a good idea to just exclude whole categories before they start.
The Avelia Horizon apparently has power cars now under 16m long.
Especially since I am not entirely sure why articulation is forbidden. Network rail is obviously not opposed.
Also don't a bunch of new french multiple units have traction gear mounted on the roof?
EDIT: (Apparently it's the Regio 2N and a bunch of Double Deck EMUs - so apparently centre of gravity is not a showstopper)
Copying recent best relevant practice in Japan and China is better than copying a compromise to historic norms in Europe that bear little resemblance to any current functional requirements.
And relevant European best practice is low platforms to enable double deck trains to be used on high demand routes like Paris Lyon.
Possibly the only saturated high speed line outside of Japan. Perhaps some CHinese routes.
Also we aren't exactly following Japanese best practice either.
Otherwise all the stations would be rigged for 3.5m train width and the classic compatible trains would have boarding bridges.
EDIT:
It's also worth noting that Brexit has rescued HS2 from having to abandon this design decision, as it appears the argument was very much going against them regarding the derogation from TSIs.
I went looking for supporting documents and found only one that supported HS2s position and several that very much supported the TSI-position.
Indeed an FOI i made to them on the actual platform height used for construction prep was replied to in a very testy manner
The DfT is engaging actively with the European Commission to identify a means to achieve step free access for HS2 within the current and future regulatory framework.
In other words they said no but we aren't willing to admit defeat yet.
EDIT #2:
Just to say my preferred solution would have been 915mm platform heights so that the classic compatible trains would have level boarding or near level boarding throughout their journeys.
Later on captive double decks could have been built with the highest achievable door height that allowed for a continuous upper deck.
That would have permitted a reasonable compromise.
But we are now where we are and I hope we don't come to regret this decision later.