Great, but it's also a goods corridor; the only way around both of those at the same time (absent massive depopulation) is to move the people, would mean building loads more houses.
"There is no available capacity without significantly impacting performance and causing a reduction in timetable resilience due to the resulting requirement for successive services to run on minimum headway"
https://www.orr.gov.uk/sites/default/files/om/wcml-capacity-...