Peter Masson wrote:

How much platform lengthening will be needed? In most cases it will
merely involve reopening sections of platfrom which have been out of
use for years, but lengthening Willesden Junction high level looks as
though it will be expensive.

That's the main culprit. Quite a lot need little work other than
stripping/resurfacing/drainage cleaning. WJ is the difficult one to squeeze
another car length out of. Even SDO (to use the inner doors only of the end
cars) wouldn't work, due to the siting of the signalling.