Great work so far, but I note you are looking at a custom double-decker railcar with what looks like a very large loading gauge. My understanding is that for most of Europe, double deckers are not used due to loading gauge limits imposed by tunnels, bridges and so on. I presume your planned routes take that into account? Is that one of the reasons your mocked journey planner app doesn’t include the UK?

The UK has adequate loading gauge for double deckers on HS1/Chunnel and HS2 (though I'm not sure if the Euston extension will support it or not).

We can run London - Europe double decker today and if we connected HS2 to HS1 it would probably be possible to run from Birmingham, Crewe and Manchester to continental Europe too.

We have them in Switzerland. Often had to change train at the border and never thought it could be a technical limitation.

The „current“ concept is for refurbished standard passenger coaches, which is compatible everywhere in Europe except UK.

The „long term“ concept is built around to be compatible with profiles UIC GB and G2, which work in most of Europe.