Brassey-Brunel? sounds familiar anyway.
And indeed the US gets it's independence at the end of the book. IIRC the tunnel went to the Azores first? also I think some of the shifting floor issues were "solved" by having the tunnel float ( underwater, obviously ). It had nuclear-powered reciprocating steam locomotives, so that is the level of reality we're talking

also the enitre tunnel is in a vacuum, which a) precludes through running and b) the insanity of a deep submarine tube with a vacuum in it boggles my mind a bit, as does the logistics of pumping out several thousand miles of tunnel.