South of Willesden Green there are still a few stations above ground before the Jubilee Line dives into darkness, and all of them - like Dollis Hill, Neasden and Wembley Park before - have twin sets of tracks running through.
On the outside, not served by platforms, are those used by battered, dirty Metropolitan Line trains operating what is dubiously described as a "fast" service to Finchley Road.
On the inside scuttle the smaller Jubilee Line trains with their purring engines and well-scrubbed carriages. All the stations are pretty close together, though, especially West Hampstead and Finchley Road. On the day I did this part of the journey it was actually quicker to walk between both stations given how, for some reason, the frequency of trains had suddenly slowed to one every 10 minutes.
Anyway, all the way down to Baker Street you're passing along the route, if not necessarily the same tracks, as that first etched by the Metropolitan Line in the 1870s, which then became the Bakerloo in 1939, and which ended up the Jubilee in 1979.
There's little to see of this pensionable heritage nowadays, thanks both to refurbishment and expediency. Kilburn, for instance, got completely rebuilt after the Second World War, in the process having its name contracted from Kilburn and Brondesbury.
It's somewhat overshadowed by the A5 passing overhead, which means that when you leave the station you kind of feel you haven't really emerged into daylight - despite having travelled in it to and from the station.
West Hampstead is a far more appealing prospect, lurking discreetly on a bridge above the line itself. As Laura pointed out in her comments on the last blog, some thoughtful renovation has been going on. Not that I'd bet most people using the station have noticed, but, like a local library or Antiques Roadshow, it's reassuring to know it's there.
Finchley Road has one of the best interchanges on the whole Underground. You can step off a Jubilee Line carriage and walk three paces - at most - to climb aboard a Metropolitan Line train. As such the station has a sort of compact, bustling feel a bit like a small ferry terminal or airport; the fact it's right on the edge of the first tunnel on the Jubilee Line compounds the air of departure and anticipation.
If you're heading south it's the last place to gasp a few breaths of fresh air before plunging underground. Equally when you're heading the north on an especially crowded and stuffy train, arriving at Finchley Road can, if you're feeling suitably emotional, feel akin to reaching the promised land. Albeit with fusty shopping arcades at the exit and a squawking dual carriageway alongside.
Finchley Road is where the Jubilee Line waves goodbye to its true Metropolitan ancestry. In the 1930s, to ease congestion on the line, a second set of tunnels were dug; but unlike the existing, just-below-the-surface ones (or 'cut and cover' tunnels to give them their official name, because they were cut into the ground then covered up), these were bored deep.
Hence the stations between here and Baker Street - Swiss Cottage and St John's Wood - lie a long way under the earth, and a long way from the original Metropolitan tracks. When the new route was opened in 1939, so were the two new stations, and the whole line all the way up to Stanmore became part of the Bakerloo.
At the same time, two old stations closed: Lord's and Marlborough Road, which I'll talk more about when covering the Metropolitan Line. Their de facto replacements have, I'd argue, two of the most evocative names on the whole network. Swiss Cottage and St John's Wood have also had much of their 1930s design restored or enhanced, notably those magical escalator uplighters which I last saw on the Morden branch of the Northern Line.
Outside Swiss Cottage there's this curious brick tower, the only element of the station that's above ground:
St John's Wood is more substantial and memorable: a sleek circular building that's retained - or recovered - much of its original elegance, despite the ugly block of flats above. I waited ages to take this photo because I wanted to get people in it; I love the way the woman appears oblivious to what looks like a deliberately exuberant pose.
The station itself was originally going to be called Acacia Road, or simply Acacia, thanks to being on the corner of the eponymous street. Granted it would have made for something even more evocative than St John's Wood, but then you wouldn't have been able to apply the latterday ubiquitous rule of the station being the only one on the Underground not to contain any letters in the word MACKEREL.
Finally to Baker Street, one of the stations on the very first underground line, indeed one of the stations on the very first underground line in the whole world, opened in 1863.
With ten different platforms it's the most sprawling station on the network. The story of its development belongs more to the story of the development of its long-forgotten parent, the Metropolitan Railway. Rising above the entrance, however, you can still see the essence of that company's breathless ambition and extraordinary imagination in the shape of Chiltern Court, designed by Charles Clark, opened by the Metropolitan in 1929, and originally consisting of half a million square feet of shops and luxury flats, some of which were home to, at one time, the likes of Arnold Bennett and HG Wells.
It's also the point at which the Jubilee Line bids farewell to the old, and plunges unblinking into the new.