The TGV-Duplexs are about 50cm taller than the UK Javelins/Pendolinos, but bridges and the like shouldn't affect them (They still have to fit between the track and the overhead power, after all).
In France with the TGV-duplex, for example, they build down in the space between the wheels. They can do this because their carriages can be 26m long, so all the "below-floor electric stuff" that we have between the wheels gets squashed up at either end of the carriage, leaving a large middle chunk where the floor can be dropped, and a second floor added above. Long carriages require shallower bends, though, and none of the TGV lines (LGVs) have a radius of less than 4km (2.5 miles), and all recent lines have a radius of at least 7km to allow for future speed increases. The UIC-GC loading gauge does allow for slightly wider trains than we have on non-HS lines in the UK as well, although the track is still standard gauge.
