Brimstone wrote:
Euston Loop? Can you tell us more about this?

Back in the days when the Bank branch platforms of the Northern Line at
Euston were either face of an island platform (like Clapham Common),
there was a scissors crossover south of the platforms to allow C&SLR
trains to reverse south to Kennington and Clapham. When the C&SLR was
connected to the CCE&HR at Camden Town and the Charing Cross/Bank
branches formed, the scissors geometry was simplified to a single
trailing crossover. Later, a link was installed between the NB tunnel
and the Piccadilly Line; this is the Kings Cross Loop, and is the only
track connection (out of two planned*) between the Northern and the
rest of the LU combine.

When the Victoria Line arrived, the NB track of the Northern Line, Bank
branch, was diverted quite a distance south of Euston into a new
alignment, and the Victoria Line brought between the two lines,
facilitating cros-platform interchange. However, the presence of the
trailing crossover was deemed to be too useful to the line, so instead
of filling in the original NB tunnel north of the divergence, a
step-plate junction was built.

Now, a NB train gets a feather to enter the original NB tunnel, travel
across the crossover and stop in Euston Bank branch SB platform, where
they change ends and head off towards Kennington and Morden.


* - the other one was a planned connection between the Bakerloo and the
CCE&HR extension between Waterloo and Kennington; later it was
downgraded to a non-track connection, and then later forgotten. I could
be wrong about this though.