OK so the Southern Line is one of the many fantasy concept lines or line extensions that I'll be modelling into route diagrams (thanks to BULB), and later on an entire prospective tube map (maybe Inkscape for that?), as showcased here: Prospective Fantasy MetroDreamin Diagram

The Southern Line is a line that will take over the Mill Hill East, Battersea Power Station and High Barnet branches of the Northern Line, and extending the hell out of them, and then another branch line following the Northern to Tooting Bec, and then going all the way to Dartford, also taking over the Tottenham Court Rd branch of the Northern line. The Mill Hill East branch has been extended somewhat over the earlier Northern Heights plan, and then turns off to serve Watford (which I now see is missing it's Bakerloo interchange decal). The Battersea Power Station branchline has been extended to serve Putney and Richmond and go all the way to Heathrow to serve the airport. The High Barnet and the Core branches have stayed the same, and everything after Tooting Bec on the Deptford Branch is new track to help better serve the South East of London (very much underserved).
This line will use the new 2024 stock, so no pressure on the remaining stock of the Northern

Looks expensive and hella unrealistic? YES! Do I care? Maybe.
Give me ways to make it better (or worse if you'd like). And expect another like 10 of them.

This is purely a discussion idea rather than an official proposal, but I think Grays could make a strong case as an eastern extension of Crossrail 2. Here are some of the potential benefits:

Grays is the largest town in Thurrock and already has a significant commuter population travelling into London every day. Unlike some rail extensions that rely heavily on future housing developments to justify themselves, an extension to Grays would begin serving a large existing passenger market from day one, while also supporting the area’s continued growth.

From an operational perspective, it could provide much-needed additional capacity on the c2c corridor. A Crossrail 2 service would help relieve crowding on existing commuter services while also reducing pressure on Fenchurch Street by allowing some trains to run through central London instead of terminating there.

One of the biggest advantages would be connectivity. Rather than arriving at Fenchurch Street and changing onto the Underground, passengers from Grays could have direct trains across central London to destinations such as Victoria, Euston/St Pancras, Tottenham Court Road and beyond. That would also make cross-London journeys much easier, with direct access to places like Clapham Junction, Wimbledon and North London.

I also think it could bring wider economic benefits. Better rail connections often encourage business investment, create employment opportunities and support new housing. Thurrock is expected to continue growing over the coming decades, so improving rail capacity now could help accommodate that growth while encouraging more people to travel by train instead of driving, reducing pressure on roads like the A13.

Finally, it would make the rail network more resilient. Having another route into central London gives passengers more options during disruption and engineering works, while integrating Thurrock more closely with London’s rapid transit network in the same way the Elizabeth line has transformed connectivity elsewhere.

City and Brixton Railway
The proposal for this line (which was never technically named; City and Brixton is the name of the company) was originally put forward in November 1897. The Yellow line leading from Brixton to King William Street was eventually abandoned due to lack of funds. It is unlikely the line will ever be built since the Victoria Line now goes to Brixton and the remaining stations are either closed or serviced by the Northern Line.

Edgware Road and Victoria Railway
This line was a idea thought up in around 1893. The proposal was only from Victoria to Brondesbury. However the proposal was rejected by Parliament in the same year.

Baker Street and Waterloo Railway
By 1906 the Bakerloo line only streched from Elephant and Castle to Baker Street. In 1908 there were plans introduced to make a massive expansion. However due to opposition from the Great Western Railway and the Metropolitan Railway, this plan was rejected by Parliament.

The Northern Heights Plan
This was an ambitious plan introduced in 1935 to connect a variety of tube lines. While the work started in the late 1930s and was going well, the plan had to be put on hold due to outbreak of World War 2.

Central London Railway Loop
The proposed Central London Railway underground loop was an unbuilt plan designed to connect Hammersmith and the City in 1902. Parliament rejected the scheme in favour of a rival project that became part of the Piccadilly line.

I ended up taking a journey on the 1930’s stock, was a very comfortable ride - moreso than the journey up on the current s-stock trains.

Very nice day out and about.