John B wrote:

Except the newly-built one at KXSP. However, since there's no
interchange on the Charing X branch, it would still be a good idea to
put one in at Euston Square (although I'd consider linking it to Warren
Street rather than Euston, given the local geography).

If the platforms are under Drummond Street then effectively the link
would be under North Gower Street. The link would include an escalator
as the H&C lines etc are higher.

And given that we are talking about splitting the lines, KXSP won't
even be on the same line.

Warren Street may be more of a problem given the Marylebone Underpass
in near proximity. On the other hand, the existence of that might make
it easier. It would mean one extra stop to make the change though
assuming most the of interchanging passengers are coming from the North.