Andy wrote:
Providing two platforms in each direction at St. Pancras would at
least prevent the hanging around and allow trains from the two
northern branches to wait their 'slot' in a platform and allow
passengers to change to the underground if they wish. It also allows
slightly easier recovery from delays, as fast trains can pass stoppers
in the station, rather than having to wait until they are further out
from the center.

As we already know, there will be no junctions immediately either side
of SPILL for some time. However, from December 2015, this will be the
new layout:


--- Down GN
Plat. /
Down TL ------------ Down TL
SOUTH NORTH
Up TL ------------ Up TL
Plat. \
--- Up GN


If SPILL had been built to accomodate 4 platforms, you would have to
build the track in either of these formations in order to maximise the
benefits 4 platforms would provide over 2 platforms:


---------------- Down GN
/ Plats. \ /
Down TL ------------------- Down TL
SOUTH x x NORTH
Up TL ------------------- Up TL
\ Plats. / \
---------------- Up GN

OR

Plat.
-------------- Down GN
/ \ /
Down TL ----------------- Down TL
SOUTH x Plats. x NORTH
Up TL ----------------- Up TL
\ / \
-------------- Up GN
Plat.


The single crossovers would, as you have pointed out, permit overtaking
moves to take place at SPILL. Assuming the signalling is of the
bi-directional variety, then the (otherwise useless) diamond crossovers
would further enhance the status of SPILL by allowing it to fully
function as a terminus for both TL services (from both directions) and
GN services (from the north). The benefits for TL passengers would be
greater as there would be more scope to limit the reduction in frequency
required to terminate services at SPILL than at SPIHL (particularly if
GN services outside the peak are restricted to KGX as it has more spare
capacity than SPIHL in that respect IMO) - ironically, it seems that
SPIHL will actually become the London terminus for TL services to/from
north of London during some weekends next year while preliminary TL
Prog. works are carried out!