All this chit-chat reminded ne of a time about seven years ago when I tried to make sense of the lines south of the river (I was middle-aged, I had time on my hands etc...) so I came up with the idea of creating a node diagram, to see if it would enlighten me to what could be done, and I came up with this. It's a little old now, and I think there are a few minor errors.
When studying the lines south of the river, I noticed something very odd. In spite of the four competing companies that built lines almost randomly across the area between the Thames and the North Downs, they sort of made a grid, albeit tilted at about 40 degrees. Then I could further simplify the network right down as this.
View attachment 190009
From this I created a new diagram.
View attachment 190010
This shows the most basic way you can travel between each node, thus tracing out routes between destinations. You should be able to work out the station names from the two-letter codes. (I can't see any errors, but their maybe a few lurking.) You can see the diagonal grid, particularly in the central section. I've popped in the Crystal Palace (High Level) and Bricklayers Arms branches to amuse. The Overground to Battersea has been missed as well as a connection between WB West Brompton and the knot of lines between CJ Clapham Junction and the lines coming out of VT Victoria. Also, take note, things have changed since this diagram was created. Apologies. And 'Tramlink'
Brockley is not shown as it is not a node, but should the high level platforms be reinstated, then it would be. It is between NH Nunhead and LS Lewisham.
Note the Bakerloo extension to Lewisham is shown, Crossrail 2 is indicated just above WM Wimbledon as per an old proposal. The DLR extension to Thamesmead isn't shown as the proposals then were to go to Barking Riverside. Neither is the West London Orbital proposal shown.
However, the diagram can be used to test the possibilities of extra services and interchanges. The situation at BT Brixton can be seen (as discussed above) as is the situation at LJ Loughborough Junction.
Perhaps a system of colours and nomenclature similar to that used by the New York Subway should be adopted.
Also, with the expansion of Gatwick, a lot of extra traffic will have to be accommodated but that is being discussed elsewhere.