On November 14, you will be able to board a Eurostar train that will get you to Paris in a little over two hours, or to Brussels in a little under. And where will you catch that train? At the gothic fantasy of London St Pancras, a station that missed out on the 20th century altogether. It’s an almost surreal conclusion to a 40-year saga.

St Pancras: interior of the Midland Grand Hotel

Back in the early 1960s, high Victorian architecture was widely considered to be hideous, fit only for demolition. Many key buildings were lost. Next in line, the highest of high Victorian, were the smoke-blackened, sinister turrets of underused St Pancras, right next to dour old King’s Cross. With the railways long since nationalised and passenger numbers falling, what need for such duplication? So, in 1966, a merged station was mooted; the wrecking ball was readied.

By then, however, the tide was turning. The Beatles and the Kinks loved Victoriana, as did the cuddly poet and conservationist John Betjeman. St Pancras was duly listed as a Grade I building, on a par with the Tower of London. But, having saved it, nobody knew what to do with it. For decades.

Now look. St Pancras has had £800m spent on it, and is set to become the most important station in the UK. Britain’s world-class engineers, Arup, have been instrumental in the whole thing, and are shareholders in London and Continental Railways, which has built the line, the new stations, everything. The red-brick turrets of the old station hotel, designed by George Gilbert Scott at his most outrageously camp, are being converted back into a hotel and multimillion-pound apartments. Judicious additions to the old building are done in knowing reference to - and sometimes outright imitation of - the Scott style.

St Pancras: construction of the Barlow Train shed, 1864

Yet, like the Victorian psyche, St Pancras has always been Janus-faced. Where Scott’s hotel frontage and station flanks harked back to medievalism, the 1868 train shed behind, by the engineer William Henry Barlow, was another matter entirely. This was Victorian high-tech, a look-no-hands marvel, for many years the widest single-span arch in existence. It is plenty wide enough and high enough to take today’s super-stretch Eurostars, but, unfortunately, it is not nearly long enough. This, along with the need for extra platforms for rail services to the Midlands, has led to St Pancras sprouting a somewhat graceless rectangular station box at the back ? from an original idea by Norman Foster, but carried out by others. It does the job, and they have separated the old and new structures with a tall, glassy transept, but it does not raise the spirits. The best you can say is that it is inoffensive and doesn’t try to compete with Barlow or Scott.

In a way, though, this back extension doesn’t matter. What does is what has been done to the old station. This is more than mere restoration, though that is impressive. At first, I was unsure about the sky-blue paint on Barlow’s great iron arches. This was their second original coat of paint, the first having been a more typically Victorian chocolate brown. But the more I see the place, the more I go with that sky blue. Together with the extra daylight flooding in through reconstructed ridge-and-furrow roof glazing, it lightens the whole place, makes it feel bigger.

St Pancras: the modern Barlow Train shed

It is a complicated picture, with many hands at work. The first part you see - the restoration of the Midland Grand Hotel at the front, and its new annexe on the western flank, both by the Manhattan Loft Company - is designed by Richard Griffiths Architects with fellow architects RHWL. That bit started later, and won’t be finished until 2010. The main work at the station is by Alastair Lansley, a railway architect whose pedigree goes back to British Rail days. He took on the Foster concept for the train-shed extension, and master-minded and delegated aspects of the conversion to others.

So, I found the retail specialists Chapman Taylor fitting out the £50m shopping centre and departure lounges in the old street-level storage undercroft beneath the tracks, where Burton beer barrels used to reside. Dirt floors are being replaced with hectares of solid timber. Plunging further downwards in one corner, you get to the clangorous box of the new Thameslink station, threaded between enormous sewer pipes; it will open in early December. Just as the St Pancras cellar used to be for beer, Thameslink used to be a goods line for coal. Victorian infrastructure is an adaptable thing.

Think of it this way: for passengers, the old St Pancras was a single-layer cake consisting of the platforms under the Barlow roof. By December, it will be a giant three-tier cake, with the roof as the icing on top. Given that all this is taking place in a Grade I-listed building, you can imagine the agonising that went on.

In the end, the decision was taken that it was better for a 19th-century station to reemerge as a 21st-century station than remain in its Victorian time warp. The big new idea works very well: they have dared to cut large slots in the original floor of the Barlow shed. So, when you’re down there, emerging from the Underground into the shops and the Eurostar lounges, you can look right up and see the great roof, seemingly 20ft higher than it used to be. It’s a view nobody ever used to enjoy.

Down there, you stroll among the cast-iron columns and fat brick arches of the old undercroft, now revealed to the public gaze. It is there because the Midland railway directors opted to bring their line proudly into town at high level, rather than in a cutting. That, in turn, meant their terminus had to be built on a plinth, making it even more imposing. It might seem obvious to use the underplatform space in this way, but early studies for the revamped station largely ignored it, cramming more platforms into the train shed and filling it with access bridges instead. Luckily, someone noticed this was idiotic.

They claim the new St Pancras will do for London what Grand Central, with its shops and restaurants, did for Manhattan. For sure, retail therapy on the scale of Covent Garden market is being built down there. As for having Europe’s longest champagne bar at platform level - that raises a smile. You used to get a stale bun there if you were lucky.

Forty years ago, while Betjeman was rhapsodising about the gasoliers in the old hotel, the architectural historian Sir Nikolaus Pevsner spotted something else. Beneath the quasi-medieval facade, he observed, lurked a very modern composition. Neither man could have guessed just how true that observation would turn out to be.