Mark Brader:
The Widened Lines... none of the books I checked
mention whether mixed-gauge track was installed; as you imply, it
would not have been needed.

Peter Masson:
H P White (Regional History of the Railways of Great Britain, Vol 3, Greater
London) states that mixed gauge was provided at first...

Hmm, that is one of the books I checked, but maybe I missed something.
I'll look again later. I have the 1987 edition, in case this varies.

Was there actually a
connection between the Met west of Kings Cross and the Widened Lines? The
tunnel was built, and a track laid through it (eastbound) in 1926 - but had
there been track in it in the 1860s?

Now there you have me.

The source I'd expect to be best for this is "London's Metropolitan
Railway" by Jackson, but I'd have to go to the library to see that.
--
Mark Brader, Toronto | "Pleasant dreams!"
| "I'll dream of Canada." -- THE SUSPECT

My text in this article is in the public domain.