Mark Brader:
Did they explain the Northern Line's right-hand running around
London Bridge? ...

John Salmon (quoting the Railway Magazine):
"The unusual arrangement of the lines between Borough and Bank
stations is a legacy from the way in which the original tunnels of
the City & South London Railway were driven between the old City
terminus at King William Street, Monument and the Elephant & Castle.
J. H. Greathead, the engineer for the construction of the line,
designed the intermediate stations, where there were no crossover
roads, with the intention of reducing the amount of walking up and
down staircases to a minimum, and arranged the two lines side by side
in plan, but at a difference of level of 9 ft. 6 in. At Borough
Station, he decided to have the up line at the higher level, and
brought the two lines to a right-handed position there..."

Yeah, this describes the layout, but it doesn't give a reason for it.
If the two lines were generally side by side in plan, putting the up
line at a higher level would not require crossing them over.

Thanks anyway.
--
Mark Brader "Computers get paid to extract relevant
Toronto information from files; people should not
have to do such mundane tasks." -- Ian Darwin

My text in this article is in the public domain.