The trouble with that idea for the UK is our loading gauge is too small to allow for such "piggy back" units.
Then with the new high speed lines there are very limited stations required along the route in between the termini, so for the UK at least I cannot see it catching on. 
I think that it is aimed more for city/ suburb commuter systems, UK gauge is 4'8" where in other countries it is 5'2". I don't think the smaller gauge would make any great difference .
:-/
No ours is actually 4' 8 1/2". "Gauge" though does not mean just for track; it also includes height and width of rolling stock and locomotives, commonly known as the "loading gauge". That is
the problem. Bridges and tunnels here, and in Europe, could not accommodate those piggy back units.

Oliver Bullied, when Chief Mechanical Engineer for the Southern Railway designed a double deck set of carriages, but due to the UK gauge he could not make them of an ideal height for loading and unloading, with that unique set being withdrawn in the seventies.
