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...

By the way, I did recheck my copy and this isn't in it (unless it's in
some non-obvious place). But...

Graeme Wall:
In Charles E Lee's The Metropolitan Line there is a photo dated Mar 1866
showing the widened lines in use at Farringdon. The tracks in
foreground are laid with mixed gauge track.

....that would be definitive. Thanks, Graeme.
--
Mark Brader | "...where did they get the sunlight in such a hurry? I know
| it can be delivered in about eight minutes, but there must
Toronto | be lots of paperwork involved." -- Michael Wares

My text in this article is in the public domain.