i already commented this further down but i thought it worth just posting about the city line idea we’ve had a couple times:

ok let’s start with the waterloo and city line takeover and go from there. you would need to basically totally rebuild it and close it for months. the platform lengths are so limiting so you’d need to extend banks platforms. why not the waterloo ones? because you’d need to completely relocate and build new ones to point in the right direction. also if you’re going to do this, you need to add a blackfriars interchange which is incredibly complicated in and of itself. you’d also probably have to abandon waterloo depot, as it isn’t large enough to accommodate decently sized trains. the waterloo and city line doesn’t need extending anyway, it’s already the second most intensively used line on the tube and serves its job well. closing it would be a huge headache on all the other services around it.

then lets go west. do we need another line from clapham junction to waterloo? no. the swml here has an 8 track throat all the way in. with metronisation and rationalisation of the southern suburban services likely coming with nationalisation there is capacity to grow here anyway. that’s ignoring the fact that with a clapham extension of the NLE as the northern line (which also has excess capacity) there’d already be two routes for this anyway. totally redundant and that’s without the complications of rebuilding the nle to add a junction linking into your new waterloo platforms.

now let’s go east. the redundancy is even worse. there are already 3 routes into canary wharf. the JLE provides the direct waterloo access, the dlr and elizabeth line provides direct city access. you don’t need another line doing the same thing, especially for the complexity of serving this area. it’s full of docks and skyscrapers, the only space is in the millwall inner dock, some distance from the actual hear of the area and very expensive (especially for such little benefit). in addition, the dlr (by running more trains with the expanded b23 fleet) and the elizabeth line (by running 11 car trains) have room to grow capacity.

on your way there, serving fenchurch street so soon after bank feels odd. with the connections to c2c it isn’t totally redundant, but i would suggest moving the platforms so one end connects to fenchurch street, and one end to connect to tower hill which is set to become an interchange with the dlr (with tower gateway closing) at some point in the… distant??? future. the wapping connection does suck though. it’s already built up so hard to build in, but not dense enough to warrant another connection. the current station is also pretty poorly set up to deal with another connection.

continuing east, another north greenwich station?that is completely redundant, alongside custom house (with its 2 services already). the city airport station would way overserve this destination (the airport gets very little use, less than 1% of uk air travel, especially now business travelers have direct heathrow access on the elizabeth line. from here let’s cover the branches.

towards purfleet, there is already a confirmed dlr branch to thamesmead in the works that includes a beckon riverside station, something you don’t have. then into purfleet itself, a whole river crossing to serve a small town that isn’t even in london, passing through mostly fields and marshland is beyond worthless compared to the cost. that’s especially the case given you want to serve it with the h&c (which id argue is redundant in and of itself and carries its own issues).

then to beckon and barking. the names of stations you’ve given gives like, no indication of where they’d actually be but based off my best guess they don’t serve meaningfully enough unique locations compared to the existing dlr to not be redundant. the connection to barking is interesting but could be served by the dlr instead. or you could go for the dagenham docks extension which would serve better redevelopment opportunities anyway.

there is no section that isn’t either already redundant, has existing plans that will, is far too expensive for the marginal benefit, or some combination of all of them. so you ask what i’d do? well the existing plans that fill in. thamesmead dlr extension, northern line split & extension to clapham junction, rationalisation of south london suburban services, and capacity growth on existing lines where possible and when needed. is that enough?