I suppose that explains the Northern Line's movement from left to
right between Moorgate & Bank until after Kennington. ...

Er, the Northern Line switches back at Borough-ish, and is due to the
way the original crossing of the Thames to King William Street (near
Bank) was built,

Correct.

though it's a very long story.

If so, it's a long story that is not given in any book I have seen and
which nobody has previously been able to provide in this newsgroup,
for example in this thread from 2004:

http://groups.google.com/group/uk.tr...b65292fba60522

If you have an explanation, I'd like to see it, and the source.

To save time, here's the background: this section was originally built
by the City & South London Railway. They first intended to build a
cable-powered tube railway from King William Street to Elephant & Castle,
then got authority to extend south to Stockwell. During construction
management changed their minds and opted for electric locomotives.
The whole route from King William Steet to Stockwell opened in 1890.

The north end of the line was built west of London Bridge, curving
sharply round under Arthur Street with the King William Street terminus
on a more or less east-west alignment. The station was too small and
in 1900 both the station and the existing under-river crossing were
closed and the line rerouted onto its present, straighter alignment
(east of London Bridge) to a new terminus at Moorgate, with new
intermediate stops at London Bridge and Bank.

Now, the original route had the two tunnels crossing over between
Elephant & Castle and Borough, and since that section was never
replaced, they still do. So it's this part that needs to be explained.
As I said in the 2004 thread, "Rails Through the Clay" just says that
it's "related to the original intention to use cable haulage", and
that's not exactly an explanation.

(The return to the normal left-hand running is between Bank and Moorgate,
on the section opened in 1900. The curve under Arthur Street approaching
the old King William Street station had one track above the other, so
it didn't constrain any other part of the line.)
--
Mark Brader, Toronto | "You can write a small letter to Grandma
| in the filename." -- Forbes Burkowski

My text in this article is in the public domain.