tubechallenge.com

Someone asked this question on another forum and I realised I didn't know the answer. Apart from the Waterloo and City line (if you can call that "mainline"), all I could think of was the section of the Bakerloo north of Queen's Park, which doesn't really count since it's still served by Overground trains. But then I discovered that the eastern extension of the Central line had previously been a mainline service:

"The extensions in the east opened in stages between 1946 and 1949. However, most
of the stations had been opened from 1856, and the extension was largely the
electrification and signalling of an existing main line railway. The only entirely new
stations were Bethnal Green, Mile End, Wanstead, Redbridge and Gants Hill. A major
train maintenance depot at Hainault was also part of these works."

https://content.tfl.gov.uk/research-gui ... l-line.pdf

(Doesn't say who the operator was - does anyone know?)

I suppose you can add the Metropolitan suburban branch from Baker Street, though it's arguable.

Any others I've missed?

The High Barnet branch was originally mainline. Trains ran from Finsbury Park to East Finchley to High Barnet, and also to Edgware via Mill Hill East. This is why East Finchley has 4 platforms, the centre two were for trains to/from Finsbury Park (now used to access Highgate Depot).

Yes, you can still see relics from the days of the GER at some of the eastern Central stations. I believe lots of the benches still say GER on them, for example. I believe the two "branches" (Leytonstone-Woodford and Newbury Pk-Hainault) were two different lines that got connected via the new underground bit.

I think the District between East Ham and Upminster is also originally London Tilbury & Southend Railway that was brought under a joint ownership between the two companies, as well as the previously-independent middle bit between Whitechapel and Bow (that probably doesn't count as mainline).

Now for the reverse question: which mainline sections were formerly the Underground?

The Northern City Line might count as mainline. And if we count the West London Line as the successor to the former Metropolitan branch to Addison Road then maybe that too?

Well, there's the East London Line. Or don't you consider that to be "mainline"?

I think the ELL's a bit of a stretch...