Your final destination isn't just in another London borough, it's in another county. Miles and miles and a multitude of stations lie ahead. It takes a while to forget about the city, though; there's a dingyness and brow-beaten air to this section of the line, a sense of it not belonging to anyone or anything, of being stubbornly tolerated by the suburbs and positively shunned by the provinces.
Perhaps part of it is down to the way the present-day service avoids most of the intervening stations between Baker Street and Harrow-on-the-Hill, keeping its head down and charging on regardless towards Metro-land. You're not encouraged to linger. You're not advised to admire.
It wasn't always this way. The line as remembered by John Betjeman didn't, for instance, run non-stop from Baker Street to Finchley Road, as now; it made three intervening stops, at stations long gone: Lord's, Marlborough Road and Swiss Cottage.
All were opened in 1868 as part of the first extension to the original Metropolitan Railway, and all were summarily closed when a second set of tunnels were built nearby and two new stations - St John's Wood and a different Swiss Cottage - opened up on the Bakerloo line (now the Jubilee).
You can still see, I think, part of Marlborough Road station from the carriage windows as you pass. Betjeman visited the ruins for his Metro-land pilgrimage, noting pointedly how the booking hall had become an Angus Steakhouse.
The previous Swiss Cottage was also the terminus of the Metropolitan line for a while, while owners, investors and politicians dithered over whether to take the plunge and turn the Underground into a fully-fledged network. After all, there wasn't much London left in the direction the line seemed to be heading.
Ultimately they went for it, taken with the notion to create London around the Metropolitan Railway rather than vice versa. Hence the huge urban sprawl towards Buckinghamshire and Hertfordshire, through what was once Middlesex, beginning north of Finchley Road.
When I was here before I'd just travelled down the Jubilee Line through a number of stations once part of the Metropolitan's domain, including West Hampstead (opened, like Finchley Road, in 1879), Kilburn and Willesden Green (1880) and Neasden (1880). Today the Metropolitan doesn't stop at any of these, rushing past on a set of tracks built alongside the others, spiriting you hastily towards Wembley Park.
Betjeman venerated this stretch of the railway, revelling in the contrasting character of the suburban villas of St John's Wood and Neasden, "home of the gnome and the average citizen", typified for him by an encounter with the affably eccentric ornithologist Eric Simms. There's little variety nowadays. Wembley Park, despite its international appellation and hugely impressive station, feels even less part of Betjeman's world now the stadium has been completely rebuilt. The surrounding area seems hugely undistinguised.
Thankfully green spaces being to poke through as you get towards Preston Road. Apparently this station has a reputation for horticultural displays. It's even won various awards. Goldfish used to live in ponds on the platforms. Sadly there wasn't much evidence of anything when I passed through.
This stretch of the Metropolitan was laid in such a rush to reach Harrow that no station materialised in the vicinity for almost 20 years. And then it was only to serve the local clay pigeon shooting site for the 1908 Olympic Games.
The trend towards openness and fresh air continues when you reach Northwick Park, a station so discreet it looks like someone's garage.
When you're waiting on the platform, with the spire of Harrow church in the distance, you do feel like you've finally put the grime of central London behind you and are shaking off the mantle of inner-city malaise. Here's the view at sunset on a Saturday evening:
Again, the station arrived long after the railway, chiefly because there was nothing here except fields when the tracks were opened in 1880. Only in 1923 was there at last a place to board the trains, initially called Northwick Park and Kenton.
Harrow-on-the-Hill had been earmarked for a station from the off. It wasn't properly marketed as both a place to live and a place to learn until after the First World War, when the 'Metro-land' concept was born and the outer reaches of the Metropolitan began to be advertised as almost the new Jerusalem.
However its name is a huge misnomer: it's not on a hill at all, and isn't really in Harrow either, but the governors of Harrow School didn't want the line going too close to their precious playing fields, hence the station's location in Greenhill. In, it has to be said, a horrible area. The station is a real dump: grotty, cumbersome, unwelcoming, and caught up in an equally miserable shopping centre. It's certainly no enticement to come and settle in the presumably now less-than-rolling meadows of Middlesex, nor a romantic gateway to the mythical heart of Metro-land. All in all it was a thoroughly dispiriting end to this stage of the journey.
Were my initially somewhat idealistic expectations about the allure of the far-flung Metropolitan about to be dashed?