The throat and line up to Neasden will never be more than two tracks, and 6 platforms is a decent amount. Other stations handle more with less.
Marylebone growth lies in:
- Running longer services / full length or at least minimum 4 cars. May come with stock renewal.
- Growth in actual demand. Is there much more that isn't served? House-building in Aylesbury (+Vale), High Wycombe, Bicester will create more. The smaller places are among the most NIMBY in the country, so I wouldn't expect material growth from Gerrards Cross etc...
- Oxford as a hub. Commercial developments (and thus in-bound), plus eventual wires, station rebuild, EWR and possible Science Park route. All of this will see activity increase at Oxford. Chiltern if wired (imagine the irony if they got to Oxford first!) would be competitive on journey times again. Parkway is still growing organically as patterns evolve. I could see a third Chiltern tph for more local service - it's a good place to terminate.
- Wires and journey time improvements. Could stock be better utilized? Or if uniform, turned quicker (per Fenchurch St) ?
- Line capacity upgrades. No major quadding is likely, but capacity projects could be opened up further between the Ruislips (4 all the way) and up to re-working Denham, the Beaconsfield loops, Aylesbury doubling (likely dead, via Monks), even reinstating the Wembley Stadium loops. Or closing the Sudburys. High Wycombe re-alignment / platforms. Plenty of ideas but all require big $.
- OOC bays. If those service do come, Marylebone would have capacity for more. But those services still hit the mainline, so that might enable more via Amersham. Perhaps MKC would be routed that way, and run much faster. No High Wycombe - MKC connection with that route however.