"2.4.4 Following the introduction of Crossrail services, the number of
platforms at London Liverpool Street station will reduce from 18 to 17.
This is to enable platforms 16 and 17 to be lengthened to accommodate
Crossrail trains; platform 18 will be taken out of commission."

That does imply that there's some period where there will be only 2
platforms available at Liv St, but that they won't be extended to cope
with 9 cars by that point? Presumably there will be a period when 9 cars
will go through the tunnel, but all surface services will still be 7?