Eurostar is the exception to every train. It's a monster of complexity because it has to work on three systems. It also does not have 550mm platform height. I think it has 750?
> some TGV trainsets
But not because of the platform height but because of stability in case of a crash. The TGVs with bogies between the cars has the same general height setup as the traditional ones.
> (If the floor is below the axle line, as with ultra low floor trams, it's really complicated.)
Sure, but that's hardly relevant here. Trams are generally set up completely different. Vienna for instance has the ULF tram and it's being phased out because such a low floor setup is just generally a bad idea for many different reasons. The new rolling stock will be Bombardier Flexity.
If the floor is below the top of the wheels, there has to be a narrow section at the car ends. The wheels have to go someplace. The lost space can be minimized by sharing a bogie between two cars.
(If the floor is below the axle line, as with ultra low floor trams, it's really complicated.)