IT'S 5.30 in the morning and I am cycling through the streets of Edinburgh towards Waverley station. Cash machines blink at me from banks, and the eerie quiet is broken only by the growl of a taxi and the occasional burst of pre-dawn birdsong.

At the station the bike rack, as always, is full. But it's not long before I'm settled on the 5.50am train to London, exhaling. This was the train I had to catch to make my journey feasible - Paris in a day on the inaugural Eurostar service from St Pancras International.

When the invitation came to join Eurostar's "green train" on its first so-called "carbon neutral" trip on November 14 2007, I leapt at the chance. But, in order to satisfy family and work commitments, I had to manage it in a day, and clearly couldn't contemplate climate-wrecking flights.

So I opted for the early morning train from Edinburgh, at the time run by GNER to get business people to Kings Cross by 10.10am. The problem was that I had been told to be at St Pancras by at latest 9.30am to allow time to get through security before departing on Eurostar at 11.01 am. So I was tight for time, despite St Pancras being next door to Kings Cross.

Since one of the main aims in opening up St Pancras was to help travellers from the north get through the Channel tunnel, this didn't bode well. But I tried not to worry about being late as I tucked into a tasty, hot bacon sandwich and checked email on my laptop via the on-board wi-fi. In the event, there was no delay, other than that caused by my heading off in the wrong direction from Kings Cross. I assumed that the way into St Pancras was through the magnificent gothic facade on Euston Road. In fact, it is round the side.

I didn't really take in the wonder of the new St Pancras at first; but believe me, it is a wonder. It's the scale of the place that's so breathtaking: the cavernous overarching glass roof, the long platforms and the huge statue of a couple embracing.

Checking in took less than 15 minutes, with staff beaming at me at every stage. And when Eurostar pulled out of St Pancras, it felt like a little piece of history. A few of the environmentalists on the train complained that St Pancras was harder to get to from their south London homes than Eurostar's previous starting point at Waterloo, but it was hard to be sympathetic.

As the train raced through southeast England, we were told the moment it reached its cruising speed of 186 mph. Using a new high-speed line, this means it is now possible to get from London to Paris in two hours 15 minutes and to Brussels in one hour 51 minutes. In theory, Edinburgh to Paris can take as little as seven and a half hours.

We arrived at the Gare du Nord in Paris in good time, but in the midst of a metro strike. All the new Vélib' hire bikes were taken, and taxis were hard to come by, so we walked through the autumn sunshine to the 17th-century decor of Brasserie Flo, where Friends of the Earth were hosting a celebratory lunch.

There, not only did the chef have to cope with providing everyone with a climate-friendly vegetarian meal, but also a request from a woman for a vegan alternative. Given Gallic carnivore culture, this did not come naturally, but it still resulted in a very pleasant meal.

Walking back to the station as dusk descended, the pavements came alive and the traffic became congested. A cup of coffee by the Gare du Nord and it was time to re-board Eurostar. There was much chat of things briefly seen in Paris.

Back in St Pancras I had time to take in its grandeur before walking the short distance to Euston, from where I was booked onto the overnight sleeper. A pint in a grubby station bar watching the inebriated getting angry and extrovert, and I was ready for bed.

The First Scotrail sleeper was the most disappointing part of the journey. The attendant was helpful and I slept reasonably well, but the compartment was battered, the toilet barely functional and the breakfast almost inedible.

Pushing my bike up the Mound as Edinburgh began to waken, I reflected on my 26-hour round trip. Was it enjoyable? Yes, for the most part. Was it really carbon neutral? No, but it was still much less polluting than going by air (see side panel for more details). Would I do it again? Yes, though I'd take the family and give myself more time - and probably go somewhere different.

After all, it's not only Paris to which you can take the train. You can get to Brussels, with its beautiful Grand Place and delicious waffles, in even less time. Or in the summer, Eurostar runs south to Avignon and the warmth of Aix-en-Provence. By onward connections, the whole of continental Europe is accessible - and further. How could I not be tempted?

FURTHER INFORMATION Book tickets from Edinburgh, Glasgow and Aberdeen on www.eurostar.com. Check www.firstgroup.com/scotrail for Caledonian sleeper tickets.

CARBON FOOTPRINT A return journey from London to Paris on Eurostar emits 11 kg of carbon dioxide per passenger.

A return journey from Edinburgh to Paris emits approx 0.087 tonnes of C02 per passenger (1.5 footprints)