Michael Bell writes
Rather than very expensively create NEW, it might be much
better value to make best use of what ALREADY IS. Things like create
interchange at the dozens of places in London where lines cross
without any interchange at all or stations just too far apart to be
really "the same place" the remnant of the railway politics of the
19th century. Places like :-

* The crossing of the North London line with the Northern
line. A pair of underground stations to be dug out. Simple
but expensive!
[...]
It all looks possible, and VERY worthwhile.

And where is the extra capacity to shift all those extra passengers
going to be found?

--
Dave