"Charles Ellson" wrote

If the same loading gauge is involved then possibly A stock would be
limited not by the tunnels themselves but by any locations involving
curves. IIRC the use of tube stock by LU was mainly because that was
the most easily available stock WRT maintenance and the associated
stock transfers without leaving the GNC line effectively in
"Drain"-style isolation.

Indeed. Until 1939 Northern City stock was presumably transferred to Neasden
for heavy maintenance, presumably dragged via Finsbury Park, the Widened
Lines, crossing to the Circle at Farringdon.. From 1939, when the line
became a detached part of the Northern Line the drags were via Finsbury Park
and Highgate (HL) to Northern Line depots. Shortly before the line was
closed for conversion for the GN Electrics a bridge on this route was
declared unsafe, and the route via Farringdon was again used to reach the
main LUL network.

Compared with modern National Rail stock Met A stock is indeed wider, but it
is also shorter, so throw over on curves is less. As pre-1976 use of the
Northern City Line by surface stock was pre-1939, and used surface stock of
the era, I do not know whether or not A stock would have fitted, or if it
didn't, whether C or D stock would have been possible.

Peter