One of the great unanswered questions concerning HS2 is what kind of rolling stock will run on it? British rolling stock is lower and narrower than standard continental stock. And standard continental stock is too tall and wide to run on existing railways in Britain. Will there be a small fleet of continental-sized stock that will be confined to the new lines? And will it be necessary to build a special, and therefore expensive, fleet of British-sized high speed trains that can run on existing routes?
This raises yet another question. Continental stock actually varies in size from one country to another. Scandinavian trains are wider than those anywhere else, which makes for added comfort, or alternatively, more seats can be fitted into the carriages. And would also be possible to build the line large enough to accommodate double-deck trains that did not have low curved ceilings like those in most of Europe apart from Finland, where the gauge is slightly wider. (1525 mm instead of 1435 mm). I cannot find any discussion of this issue.