Mike Harvey:
There was a marvellous 1972 SF novel called "A Transatlantic Tunnel
Hurrah!" by Harry Harrison, (an Anglophile American) in which he
depicts a kind of parallel-universe situation

("Alternate history" is the usual term.)

where the British Empire
still exists and contains the American colonies. ...

It was called "Tunnel through The Deeps" in the USA.

I liked that the trains were nuclear-powered.

An earlier novel of a transatlantic tunnel was "Der Tunnel" by Bernhard
Kellermann, filmed under the same title in 1933, then remade in English
in 1935 as "The Tunnel" (British title) or "Transatlantic Tunnel" (US
title). I've seen this. I was amused to see that they have the tunnel
breaking into a volcanic zone halfway across the Atlantic, which was
unknown at the time!

I was also amused in that the excavation of the tunnel apparently produces
*no* spoil...
--
Mark Brader "How diabolically clever: a straightforward message!
Toronto Only a genius could have thought of that."
-- Maxwell Smart (Agent 86)

My text in this article is in the public domain.