In order for rail to not only be a viable alternative to road, but indeed to be the superior choice, it needs both the coverage between sources and destinations as well as a service level this is both frequent enough and with a suffiently wide operational time range in order for it to be useful. There is no utility in a transport scheme that lets you get to work but offer no services to get you home afterwards.
The solution to these problems I feel is RingRail, a series of orbital rail services that link the radial routes eminating from London, acting as both feeder routes to the main arterial radial routes to both reduce car usage as well as relieve the limited central area's rail capacity, by enabling passengers and freight to bypass the London-based routes, leaving the capacity for movements that actually originate or terminate in London and are not just merely passing though.
Ring 1 would be the existing circle, athough I would recommend the implementation of another of my ideas, the Revised Circle Line. The main purpose of this ring is, as currently, to link the London Terminii north of the Thames.
Ring 2 would be the new London Overground service, largely as proposed. Essentially, this ring serves the Greater London area.
Ring 3 serves the area beyond greater London, roughly as a Rail equivalent of the M25. The proposed route is: Staines, West Drayton, Uxbridge, Denham/Harefield road, Harefield, Rickmansworth, Watford, St Albans, Hatfield, Hertford, Broxbourne/Hoddeston, then new alignment to Epping, then new again to Romford, Upminster, Northfleet, Longfield, then somehow across south London, probably via Bickley, Norwood Junc., Down to Tattenham corner, then new to Epsom, then given the lack of better options, new route to Staines
Ring 4's purpose is essentially a bypass route for the south east, and would be: Reading to Oxford, Bicester, Calvert, Bletchley, Bedford, then new to St Neots, from there on to St Ives, down to Cambridge, Then to Stansted through to Witham via Braintree and Dunmow, then down to Battlesbridge/Wickford, then new to Pitsea, on to East Tilbury, then across the river to Higham and Strood, then along the line down to Paddock wood through to Guildford, then along to Reading.
As will be seen in a moment, interchanges will be planned where these orbital routes meet the main trunk routes, which are the primary heart of the plan, as they permit the maximum flexibility of the network as a whole. To accomodate this in the most cost-effective manner, the stations on the routes will have their platforms on passing loops, which enable express trains to pass the all stations services. These are crucial for the routes to be useful for bypassing the central London area whilst maintaining a frequent enough stopping pattern to entice travellers out of their cars. The only alternative would be to quad track the entire route, which given it's length would probably initally prohibitively expensive.
RingRail is not only about the orbital services though, integration with the existing radial services is essential for efficient flow of traffic across the network. Whilst this is primarily a case of revising the existing services, additional segregated tracks are suggested on all routes as are already present on the the WCML ( InterCity, Suburban, and Metro ), however at the very least segregated Intercity and Suburban lines are required. The existing Thameslink and Crossrail are the primary radials, and to these new services would be formed via Euston to Waterloo, as well as the Finsbury Park to Moorgate line extended to either Cannon Street or London Bridge.
Thus the major interchanges between the orbital and radial routes will be:
Nobody would want to travel over halfway around the orbital routes (as it'd probably be quicker going the other way!), so overlapping services are envisaged for each ring ( the complementary services runnning over the other half of the ring are not listed for brevity ):
Each of these radials will run at least inner and outer services ( though other services would be in additon to these ), reaching out as far as major interchanges with each of the rings, eg. Thameslink would serve all stations Bedford to St Albans, then fast to Gospel Oak, then all stations in the central section, then fast to Norwood Junction and then all stations to Redhill. The inner service would run all stations between St Albans and Norwood Junction. Likewise, Crossrail would run all stations from Reading to West Drayton, then fast to Paddington, all stations to Liverpool Street, and then fast to Romford, and finally all stations to Witham. Accordingly, inner services would run all stations from West Drayton to Romford.
Stopping services will obviously serve the stations in between, but the above are the ones I feel fast services should serve. The Suburban services can further be enhanced by the addition of a third set of tracks for metro services along the radials out to the Ring 3 interchanges.
So there you have it, the master plan. Whilst it seems infeasible due to the amount of station alterations needed and the amount of track quading required, the network as described here would be extremely well used if it existed given it's flexibility, as getting anywhere within it's scope would be quick and easy with minimal connections, similar in fact to the design ethos of the London Underground's network.
As they say, a picture says a thousand words, so here are some diagrams of what is beig proposed.
Ring 1
Ring 2
Ring 3
Ring 4