"Mark Brader" wrote

The Widened Lines are, of course, what is now the part of Thameslink
that runs alongside the Met/Circle/H&C, plus the now closed branch
to Moorgate. They were created to allow capacity for trains running
through between the GNR or Midland and the LC&DR (like Thameslink)
and from these railways to Moorgate. But none of the books I checked
mention whether mixed-gauge track was installed; as you imply, it
would not have been needed.

H P White (Regional History of the Railways of Great Britain, Vol 3, Greater
London) states that mixed gauge was provided at first on the Widened Line,
and implies that GWR Broad Gauge trains used the Farringdon to Moorgate
section. He also states that mixed gauge was provided (but may never have
been used) between Kings cross and Farringdon. 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?

White does appear to have consulted original documents, though he does not
explain which document led to which of his statements.

Peter