Solario wrote:

The CX & Embankment Northern Line stations must be very close to each
other.

Not as close as some (e.g. Covent Garden & Leicester Square). From
recollection the CX Northern line platforms are on the northern side of the
Strand whilst the Embankment ones stretch under the Thams (I think the loop
that formed the northern platform does). But one can still see the light at
end of the tunnel when looking north.