Obadiah Jones wrote:
Or perhaps I should say 'what was the motivation for building it'?

I only ask because London Bridge is a mere ten minute stroll from
Cannon Street. Presumably Cannon Street trains have always passed
through London Bridge (apart from those heading towards Charing
Cross). So why go to the expense of building viaducts from the
Borough Market junction, bridging the river, and building a
terminus station at Cannon Street when it's virtually within
spitting distance of a much more significant station at London
Bridge? Was it perhaps intended to extend the line further north at
some point?

Cannon Street station opened in 1866, and for the next 50 years most
trains were reversed there before continuing to Charing Cross (opened
1864). H.P. White wrote in 1963; "In 1904 between 5 and 6 p.m. on
weekdays 25 down trains and almost as many up passed through London
Bridge and all but 2 or 3 had to be reversed in Cannon Street." And
these were all steam-hauled! In those days, Cannon Street had more
passengers than Charing Cross, so the demand was clearly there.

Also, the (horse-drawn) traffic congestion along the Strand and Fleet
Street in the 1870s and 80s was such that a shuttle service at 5-minute
intervals was run on an additional track between Charing Cross and
Cannon Street. (The Circle Line wasn't completed through Cannon Street
until 1884.)

--
Richard J.
(to e-mail me, swap uk and yon in address)