Before cheap flights, trains were the economical way to discover Europe and its foibles. Personally, I enjoyed the old fuss at border crossings. By the time I was 18, I had memorised those warning notices in the carriages: Nicht hinauslehnen; Defense de se pencher au-dehors; E pericoloso sporgersi.

Those three different ways of saying ‘don’t stick your head out the window’, one bossy, the other pedantic, another gently pleading, summarised the nice subtleties of national borders that were philosophical as well as political.