Stop the train, I want to get off...

My friend John was at his desk, pretending to work, when he called me to discuss his forthcoming trip to Rajasthan. "Where are you?" he asked. I was pretending to work, too: rambling beside the first significant new railway line to be built in Britain for a century. Stage one of the Channel Tunnel Rail Link carries its first fare-paying passengers through east Kent tomorrow. At the promised top speed of 186mph, I explained, the exquisite detail of the North Downs and the valley of the Great Stour will elude those on board. So my mission, this implausibly sunny day in late September, was to write about what they will be missing by travelling too fast.

"The day that a train in Britain goes too fast will be the day that Hell freezes over," retorted John, and changed the subject from eastern Kent to western India. Perhaps he has spent too long commuting on the London to Brighton line, completed 162 years ago this week, and on which even the fastest expresses average no more than 60mph. From tomorrow morning, the highest speed at which passenger trains run in this country will increase by 50 per cent - and Britain will become a blur.

Brief encounter? It will be for passengers on Eurostar trains to Brussels, Paris and beyond, who will waste barely a quarter-hour covering the 46-mile link between Fawkham Junction, near Gravesend, and the mouth of the Channel Tunnel. Trainspotters' notebooks will be bereft of locomotive numbers, while those on board will miss out on millennia of history. Yet Kent will also gain an attraction, a strangely beautiful scar.

Looking at the railway network, you might conclude that the one English county that does not need an extra rail link is Kent. But the tangle of lines is strictly 19th century. The Victorian engineers would be amazed that the railways they built to carry commuters in one direction, and hop-pickers in the other, are being used to bear trains capable of travelling at 186mph.

At last, the lamentable reputation that Britain's railways have acquired may begin to wane. As a civil engineering achievement, the Channel Tunnel Rail Link is not quite on a par with the Pyramids or the Panama Canal, but it genuinely looks good on Kent. The twin tracks rest upon mattresses of ballast, cradled in concrete and guarded by catenaries that carry the overhead power lines. The link zips through Kent, a county laced with communications links ever since the Romans built Watling Street between Dover and London, nearly two millennia ago.

The new line follows a more southerly trajectory, beneath the lip of the North Downs, but it hardly amounts to an act of vandalism perpetrated upon the Garden of England. Hitherto-untouched countryside this is not: the link largely follows the course desecrated 20 years ago by the M20 motorway, which hums constantly and disharmoniously at anyone walking the line of the railway line. Yet unlike the sordid six-lane stripe of concrete that rumbles across Kent from London to the Channel, the new line carves a slender, delicate course. It doesn't exactly melt into the landscape, but it is, as the French would say, sympathique.

The French could say quite a lot about the way the nation that pioneered the railway has, at least in a token way, finally caught up with the rest of the Continent. The first Train à Grande Vitesse departed from the Gare de Lyon in Paris in 1981. The tentacles of Europe's finest high-speed network have since spread across France. The last link, east to Strasbourg, is being built à grande vitesse.

Across the Rhine, Germany is racing to trump the French, while Amsterdam is soon to be linked to the Continent's high-velocity network. Italy and Spain, too, are speeding ahead. Meanwhile, British travellers have been lumbering around for decades on what the French call "classic" trains (John could probably suggest a different word). With our rolling stock a laughing stock, no wonder the no-frills airlines are thriving.

The "classic" train I caught from Waterloo to Ashford lost a skirmish with a couple of Eurostar trains on the old line through Kent, and arrived 15 minutes late. Once the expresses are taken out of the equations, congestion in the complex commuter network in Kent should improve, or at least move. Because the second stretch of the Channel Tunnel Rail Link will not open for another four years, passengers from France still face an ignominious end to their journey. Just as the train is rocketing towards London, the line suddenly veers left south of Gravesend, and shoots along a link line before slowing to a crawl to join the queue of Connex South-Eastern trains through Swanley and St Mary Cray, infuriating domestic travellers who watch the sleek white-and-yellow train glide by.

The woman at the tourist information centre in Ashford confessed she found it hard to be overjoyed by the track that has torn up the south of the town. No one is allowed to board Eurostar trains in Ashford unless they are leaving the country. So hundreds of empty seats will whizz to Waterloo while commuters are crammed into antiquated stopping trains.

This particular dismal universe is difficult to imagine on a sunny day as you wander up to Charing, a village on the south-facing escarpment of the chalk downs that once ran, like a crooked spine, right through to France. Charing has seen Kentish life. It began as an Iron Age habitation around a spring where the chalk of the North Downs perches on impermeable clay. Later, it became a way-station for travelling clergy, being a day's journey from Canterbury towards London; the remains of the Archbishop's Palace have been incorporated into a working farm.

On a lazy September afternoon, Charing seems a backwater - yet it is in the middle of some of the great human highways through Kent. The chalk ridge sheltering the village was an Iron Age expressway, while down in the valley the original railway and M20 crowd in with the Channel Tunnel Rail Link. For the pedestrian, there is only one sensible choice: the Pilgrims' Way, a long-distance straggle from an age when highways needed bear only humans and animals.

Some cynics maintain that the Pilgrims' Way is the invention of an imaginative cartographer, who decided to liven up his patch of the Ordnance Survey. He plotted a fanciful cathedral-to-cathedral trail from Winchester to Canterbury, they say, and ascribed it to early Christians. But the trail that leads north-west from Charing, paralleling the new rail link, feels comfortably ancient: a sunken path, enveloped by hedgerows and trees, like a verdant tunnel.

After a couple of introspective miles, a five-bar gate unlocks the countryside, an instant transformation from mellow woodland into broad, hazy meadows of the sort previously thought to exist only in Provence and Tuscany.

This stretch of the Pilgrims' Way has been hijacked by yet another thoroughfare: the North Downs Way, a 125-mile National Trail that carves an entrancing path through the stockbroker belt and the green belt, skirting suburban gardens and the Garden of England.

I left the track clawing across the chalk to find the source of the river that created the valley below. It turns out to be a duck pond on the edge of the village of Lenham. The Glebe Pond, where a spring rises, feeds the embryonic Great Stour, the river that created the corridor for the new rail link. The Great Stour flows down to Ashford, then carves through the North Downs to Canterbury and spills out into Pegwell Bay - a location notable in the chronology of cross-Channel travel, as one end of the ill-fated Hoverlloyd hovercraft link to Calais.

Time to survey at close quarters the 21st-century solution to ensuring the Continent is not cut off. Lenham station, to the south of the town, has a Railway Children tranquillity to it, with the occasional visit from a train almost as old as the track. Almost alongside, the French might build a new station called "Lenham TGV", but in Britain there is no such halt on the high-speed line. Indeed, where is the high-speed line? According to my map, it rips through the hamlet of Sandway. But you can wander along the lane with only the newly planted trees and shrubs (part of an order of 1.2m plants placed by the railway builders) showing evidence of recent disruption. This is one of several sections of lines set in tunnels to minimise the visual impact.

If you insist on seeing the line up close, you can clamber along a right-of-way and down to the trackside. Behind the barbed wire, the line is of a higher order of magnitude to the branch railway nearby. You can see the line stretch out for perhaps three miles into the autumn haze, a distance that will be covered in less than a minute. Make that 5km; metrication is infiltrating Britain's rail network, with the distance markers on the new line calibrated in kilometres.

This is the ideal place to survey the scale of the task that the builders faced: to thread a path straight and flat enough to allow the Eurostar trains to show their paces without trampling on too many backyards. Hundreds of bridges have been built, some of them crowned with metalwork that looks strangely similar to the profile of a Eurostar train, with a streamlined wedge at each end.

The old road to Maidstone meanders across the new line to Harrietsham, whose 18th-century almshouse was (curiously for this inland location) built by the Worshipful Company of Fishmongers. The town lingers in the past, with a pair of ancient Shell petrol pumps and a fading mural proclaiming Hovis to be "dainty bread".

The future is nuzzling up against Harrietsham. A few hundred yards away, the new line and the old road become concurrent. With a sharp flick to the left, the Maidstone road dives beneath the Channel Tunnel Rail Link, and sneaks off to the olden days again - in the form of Leeds Castle, Kent's magnificent answer to a Loire château.

I paused beneath the viaduct that carries the line off to the Medway. "Who'd A Thought It?" reads a sign pointing to a 16th-century inn of that name. Who would have thought it, I mused: in just a few days, this silent sculpture of steel and concrete would carry the fastest land vehicle in Britain. Just then, two things happened. John took a moment out of his busy afternoon to call back and ask about cheap flights to Delhi. And a Eurostar train, which I later learned was carrying travel agents on a familiarisation jaunt, appeared from nowhere. Like a Valkyrie à grande vitesse, it howled past at 186mph. The noise drowned out exactly what John said, but it sounded more robust than "Who'd A Thought It?".

Hell is about to freeze over, and the Continent is about to get closer.

TRAVELLER'S GUIDE

The course of the Channel Tunnel Rail Link is shown on new Ordnance Survey 1:50,000 maps. The official website, www.ctrl.co.uk, has a neat set of maps at a scale of 1:250,000.

The cheapest Eurostar (08705 186 186, www.eurostar.com) ticket on the new link is on the weekend-only £35 "Nightclubbers" ticket from London to Calais, Lille, Brussels or Paris.

Normal return fares start at £55 return to Calais and Lille, £59 to Brussels and Paris, if you stay away at least one Saturday night.

Who'd A Thought It Inn, Grafty Green (01622 858951).