On Sunday, 28 August 2016 19:05:57 UTC+1, e27002 wrote:
Guy,

It is intended to be a four track mainline. The fasts to Birmingham
would use the fast pair. The stopping service to Aylesbury, and
Milton Keynes would use the slow pair.

If you had read NR's latest Route Study, you'd notice there is NO mention of a four track railway - even to 2043.
Yes, some services are likely to go to OOC, both from the Mail Line & the heartlands stations, which would free up a platform or two / paths from Northolt top Marylebone. This won't help the Metro stations as there is nowhere to start a metro service from - the Ruislips will still need to handle all the services....
Chiltern have no interest in more Metro services as TfL won't pay for them.