In terms of popular legend, cultural impact and poetic evocation, no other Underground line comes close to the Metropolitan. Admittedly a lot of this is thanks to John Betjeman, but even before his televised paean to 'Metro-land' in 1973 a mystique seems to have surrounded the line, one that had been pointedly cultivated for almost a century.
I guess some of it was due to it being simply the world's first underground railway. It naturally commanded influence upon all that followed. The Paris Metro, for instance, took its name from the line, its full title being Chemin de fer métropolitain. But there's also a unique character to the line that is, I reckon, still palpable today.
You sit in its trains and you're transported back decades, both in a literal sense - the carriages hail from the early 1960s - and an emotional one: from out of the depths of the city of London the line hauls and heaves its way to the daylight and open spaces of Buckinghamshire, through landscapes and environments dotted with relics of days gone by.
Feeling suitably stirred, I decided to travel the Metropolitan in a different manner to those lines I'd covered so far. It's the only major line to not cross London from one side to the other; rather it begins in its heart and heads one way and one way only: north west. Hence I thought I'd start in the centre of the city (the oldest part of the line) and voyage outwards, in part to mirror the line's expansion, and in part to shamelessly follow the same hallowed path as Betjeman.
That meant embarkation at the rather grim vista of Aldgate:
Outside might not be that preposessing, but inside things are very different. Aldgate's a right roustabout, to be honest, with trains beginning and terminating from all sorts of platforms, besides stopping off seemingly at whim en route around the Circle Line.
There's little apparent logic to what goes on here. You can sit in a carriage at Aldgate for 15 minutes and have no idea when or if you're likely to move, all the while watching trains steam in and out of an adjacent platform, oblivious to your plight. I know this, because it happened to me. The place must operate as a kind of "resting" point for Circle Line trains as well. Then there's the fact that everything and everyone has to wait if there's a Hammersmith and City line service about to join the tracks just north of the station.
Suffice to say if you're not in a rush, and you're fascinated by all this kind of thing, watching Aldgate at work is, I have to confess, quite thrilling. It's a grand station as well, dating back to 1876 when the original Metropolitan line began inching its way south from Farringdon. You descend to the open-air platforms via a string of capacious staircases, and then the fun begins.
It wasn't always the start/end of the Metropolitan, though; trains originally ran onwards along what is now the East London Line towards New Cross, while you could catch a service in the opposite direction running all the way to Richmond. Such was the scope of the original, swagger-esque, pre-London Transport Metropolitan.
Aldgate also has the distinction of playing a crucial role in the Sherlock Holmes story, The Bruce Partington Plans. Holmes uses his knowledge of the points system north of the station - naturally - to deduce, among other things, a particular dead body hadn't been pushed from a carriage but had fallen from the roof. Comical confusion, vexatious connections, a whiff of murder: is there nothing this station can't do?!
Liverpool Street opened as Bishopsgate in 1875, later renamed along with the new mainline terminus in 1909. There's precious little personality here; endless revamps and ongoing renovation have stripped the station of scant sense of history.
Both Moorgate and its neighbour, Barbican, date back to 1865 and were the first 'new' stations to be added to the original Metropolitan service. I'd already visited Moorgate when travelling along the Bank branch of the Northern Line.
Barbican, by contrast, was something of a revelation. Squashed in amongst the titular estate, it has a somewhat imposing entrance...
...which gives way to a cavernous interior open to the skies and blessed with enough light to allow everywhere to bathe in a rather wistful early evening sunset:
You won't find Barbican on old maps of the Underground; it originally went under the name Aldersgate Street, later shortened to simply Aldersgate. There are no traces of this former moniker to be found today, though - unlike the next station along the line, Farringdon:
Here's where you venture furthest back through time. It was near here, on 9th January 1863, that the terminus of the original Metropolitan Railway was opened. Farringdon Street, a short distance from present-day Farringdon, was one end of line that ran from Bishops Road (near Paddington), a distance of four miles.
Conceived as a means of linking London's various north-serving mainline stations, and a way of alleviating road congestion, the world's first underground was, history implies, a shambolic yet dignified affair prone to smog, flooding, and slander. Yet almost immediately there was talk of expansion, and after just two years the station was moved to its present location when the Metropolitan opened its extension to Moorgate. It was then temporarily renamed Farringdon and High Holborn in 1922, a name that is still emblazoned across its stirring exterior:
To leave Farringdon on a train heading west is to travel through tunnels built almost 150 years ago.
Next, King's Cross St Pancras - again:
The Metropolitan platforms were moved in 1941 to make the business of changing lines easier; now, in 2007, the whole station is still being tinkered with to achieve much the same ends.
Euston Square, the interchange-with-Euston-that-isn't, opened as Gower Street. It's a poky, horrible station that now lives entirely below ground. The sooner they build that connecting tunnel with Euston mainline station the better.
Great Portland Street, conversely, is represented by an impressive standalone building into which you cheerily pass and, after navigating some stairs, find yourself standing on platforms that look and feel straight from the late 19th century.
Unlike deep-level Underground lines, the Metropolitan shares with the Circle, District and Hammersmith and City a penchant for platforms either side of one broad stretch of railway (as opposed to each track - and therefore each platform - using a different tunnel). This creates a sequence of vast cave-like stations, reeking with an atmosphere that's sometimes magical, sometimes miserable.
Yet another latterly renamed station, Great Portland Street was opened as Portland Road.
And so, once more, to Baker Street.
You don't pass through the original Metropolitan line platforms here either; they're now used by the Circle and Hammersmith and City lines. When the first westward Metropolitan extension was built to Swiss Cottage in 1868, the present day platforms were added. And it's from these that I was to begin my journey into Metro-land.