In message , at 13:33:12 on Sun, 3 Jan
2010, David A Stocks remarked:
The OPs suggestion that the Jubilee Line should have built further
north in order to facilitate an interchange with the existing Northern
would probably have meant putting the Jubilee Line plaforms under the
river.

Only if the two sets of platforms aren't allowed to overlap. Looking at
the surface maps, and drawing a line between Waterloo, Southwark, and
London Bridge, it does seem very likely that the Jubilee line platforms
are south of Southwark St.

This would have been a bad interchange with the main line, and may well
have run into other problems because ISTR one of the reasons for
building a new southbound (rather than northbound) platform tunnel was
that the new tunnel had to be threaded around the foundations of both
the current and pre-1830 London Bridges, not to mention the old C&SLR
tunnels to King William Street station.

The C&SLR tunnels are above the Northern Line tunnels, so don't have to
be "threaded around". They also curve sharply just beyond the station to
cross the river west of the bridge, whereas the Northern Line goes to
the right.

It would also surprise me to find that the Northern Line platforms went
under the river, so that sets some kind of bound upon the southern end.
From the "liftshaft building" to the river bank is 400ft, so that's
about seven cars.
--
Roland Perry