The idea of splitting up the Northern line isn't crazy. It originally was two separate lines — the City and South London Railway (running from Morden via Bank to Camden Town) and the Charing Cross, Euston and Hampstead Railway (running from Kennington to Edgware via Charing Cross, with a spur to Archway from Camden Town).
The name "Northern line" came into existence in 1937, bolting together all the various bits that confuse tourists, and Londoners who aren't paying attention, to this day. Wouldn't it be easier to untangle it and have simpler routes?
That's something Transport for London have been wanting to do for a while — and a document from July 2015 (PDF) proposes the Northern line will be split by April 2023. By getting rid of the bottleneck at Camden, where lines cross over, TfL thinks it can run 33 to 36 trains per hour. At the moment, only the middle sections approach that kind of frequency.
Before that can happen, Camden Town station needs significant upgrade works, as does Kennington, because these stations would become major interchanges between the two lines.
So we've got two new lines coming. Where would they run?
It seems almost certain that the Edgware branch would run via Charing Cross and terminate at Kennington. Or, quite possibly, terminate at Battersea Power Station — the way TfL has drawn the map of the extension it's quite clearly running on from the Charing Cross branch. The new stations at Nine Elms and Battersea are scheduled to be ready by 2020.
The other line would run from Barnet via Bank down to Morden.
Now we're getting to the interesting bits. What would these new lines be called?
It's tempting to keep the Northern line name for the Edgware-Kennington/Battersea line and call Barnet-Morden the Southern line. This, however, is a bit boring (though still better than naming a line after another member of the royal family).
How about following the portmanteau method, like the Bakerloo? We could have the Burntbat line; the Tott-Toot line (after Totteridge, not Tottenham Court Road); the Colinloo? Or what about the King's Elephant line; Borough & Barnet; we could rename Charing Cross and get the Hampstead & Trafalgar line.
We can also foresee a campaign for the Mornington line, after the marvellous Radio 4 game. Anyone who suggests the City line gets banished for sheer dullness.
The current tube map is a big ol' mess. We already have 11 solid colours, plus four more in stripes (think DLR and Overground) and that's before Crossrail the Elizabeth line turns up in purple.
One of the lines will probably stay black; probably whichever keeps the inevitable Northern line moniker. But what happens to the other one? There could be an option of going for black and white stripes; but striped lines are generally kept for more rail-like services.
There aren't that many colours left. We could have a light green, maybe. Or a darker or lighter shade of purple than Crossrail the Elizabeth line Crossrail. Whatever happens, it'll look bloody awful. Harry Beck never imagined the tube map would be so busy — time for a complete overhaul?