Daft plan? Go on then:
Rebuild all the lines into London subsurface from some distance out.
Define a "Metro" limit. Roughly:
LTS: Tilbury and Basildon
GE: Shenfield
WA: Chingford, Harlow Town, Hertford East, Enfield Town
GE: Hertford North and Welwyn Garden City
MML: St Albans
WCML: Hemel Hempstead (and St Albans Abbey)
Chiltern: High Wycombe (and Amersham)
GWML: Maidenhead, Greenford, Windsor
South Western lines: Hampton Court, Dorking, Shepperton, Chessington South, Woking, Guildford both ways, Windsor*, Weybridge, Kingston and Hounslow loops.
South Central lines: Horsham, Guildford, Tattenham Corner, Caterham, Beckenham Junction, Sutton Loop, outer South London Line
South Eastern lines: Hayes, Sevenoaks, Gillingham, Bromley North.
All to be paired up and connected through. All subsurface, dedicated tracks. Interchange stations at all limits. However, see part two.
So, that. Plus:
Reengineer all LU lines. Subsurface and tube. To be able to take 200m long singledeck euro-width and height AC OHL units. As would all lines mentioned above. Where appropriate, lines mentioned above to be merged into LU lines rather than new tunnels. Circle to be a pair of grade separated lines with no conflict. All busy junctions grade separated.
All platforms to be straight, with step-free access and platform edge doors throughout
Busiest London Bus routes to be turned into subsurface trams or elevated light rail, connected to either the Croydon Tramlink or DLR.
I realise that subsurface trams would be tricky. Well yes. No-one said this was to be easy.
Similar schemes for Birmingham, Manchester, Liverpool, Nottingham, Newcastle/Sunderland, Glasgow, Edinburgh.
*through to GWML via tunnel