Many urban 'road-widening' and 'redevelopment' schemes of the 1960s 'sacrificed' lots of houses, and presumably that was thought worth it for the gain?
For better or worse, the "get it done" attitude of the 60s is long gone, certainly when it comes to railways. You have to have several stages of consultation, submit a Transport and Works Act Order application, then wait until the Minister decides to take it off their "to do" pile (the TWAO for Manchester Picc platforms 15-16 has been sitting on that pile for years)...
In short, no chance.
How many platforms at a terminus station are needed to service a double-track line running at peak capacity?
The Victoria line manages to operate 36tph using 2 platforms at either end of the line, without so much as a grade separated crossover.
The problem is the turnaround time, which is linked to the diversity of routes served. It's only possible to achieve that sort of frequency when all trains do the same route, so you can do your turnaround in 1-2 mins because you don't need to worry about waiting for your departure "path".
So, the answer is that "it depends" - on how many different routes and calling patterns you want to serve, amongst other things. I think it would be safe to say that Marylebone's 6 platforms are not overly generous, but needn't be unworkable.
I mentioned about the possibility of an additional tunenel to the west of existing- is that a possibility?
In short, no. Not unless it were a deep-level bored tunnel. Just as one example, MCC raised a stink about the fact that Network Rail wouldn't let them build an extension on top of the existing cut and cover tunnel that runs along the edge of their grounds.
You would have to compulsorily purchase thousands of properties... It's just not happening.
And if the Aylesbury services were 'incorporated' somehow with the Met? I recognise the 'territorial' issues related to TFL and how that has played out in relation to eg Watford, the Croxley link, Bakerloo 'truncations' at Queens Park or Harrow & Wealdstone and 'traction
Operationally that would make some degree of sense. The difficulty is that TfL are highly unlikely to want to take on the liability of an additional loss-making stretch of line, that stretches far beyond Greater London.
I think a deal
could be made, but it would involve the government having to pay TfL a lot of money, not least for whichever electrification option is chosen north of Amersham.
Could there then be focus on more-or-less a straightforward up-and-down Marylebone- Birmingham catering for a pattern of regular express/ semifast and stoppers, interleaving with Oxford- Birmingham and Birmingham services as London trains fall off north of Banbury. Sorry if I'm not putting that clearly- I'm not a 'professional'!
Why would you extend all services to Birmingham? It simply doesn't make any sense, even if it were possible (and that's not a given - take a look at the graph north of Aynho Junction).
Good point- thinking of how Thameslink transformed Blackfriars- Holborn Viaduct/ Farringdon- Kings Cross suburban etc -where might Chiltern trains head to through and beyond central London?
It seems very unlikely that Marylebone will ever see a "low level" set of platforms. If anything big happens to the services on the Chiltern lines, I would expect it to be for a limited number of trains to be diverted into Old Oak Common once that part of HS2 is completed.
In the distant future, I suppose there's potential for some of the Crossrail Paddington LL/OOC terminators to be extended to join up with Chiltern OOC services. But that would require a hefty amount of grade separation at OOC and South Ruislip to be feasible, not to mention electrification from OOC to High Wycombe/Gerrards Cross or thereabouts.