It wasn't the speed, though that was extraordinary and almost felt like flying. It wasn't the restored St Pancras station, a sumptuous and inspiring blend of Victorian Gothic and hi-tech (if you don't mind the 27ft-high snogging couple, a bronze statue intended as a meeting point). It wasn't even the convenience of shooting from central London to central Paris in a little over two hours, the fastest it's ever been possible.
No. It was the curious feeling that perhaps you really were having your cake and eating it – environmentally – that made the first trip on the new, fully high-speed Eurostar passenger train yesterday so different. For this, say the operators, is the world's first carbon-neutral train service. Never mind the speed. Take it to Paris, and you are responsible for only a tenth of the carbon-dioxide emissions you would account for if you went by plane – and those emissions that are down to you will be neutralised by Eurostar in carbon offsetting schemes, at no cost to yourself.
Can this be true? Comfort, convenience, long-distance travel and minimal damage to the planet? Guilt-free zooming around? The green pressure group Friends of the Earth certainly thinks so, and has joined in a partnership with Eurostar to endorse the new service, which it sees as a model for environmentally friendly, low-carbon mass transit schemes everywhere; for travel with a low-carbon footprint.
To make the latter point, the executive director of FoE, Tony Juniper, and Eurostar's chief executive, Richard Brown, ceremonially named the train that hauled the inaugural service, the 11.01 from St Pancras International to Gare du Nord: they christened it Tread Lightly.
The company claim, using independent research, that a passenger on the train from London to Paris is responsible for 10.9kg of CO2 emissions per trip, compared with 122kg of CO2 for the air journey. Friends of the Earth accepts Eurostar's figures, Mr Juniper said.
"Today is a landmark day for green travel and the first step to making short-haul flights a thing of the past," he said. "Eurostar shows that short-haul rail travel is not only better for the environment than flying, but that it can also be quicker, easier and more comfortable as well."
How times have changed! When railway boffins, as we used to call them, first dreamed up the high-speed link, about 20 years ago, what they dreamed about was Faster. Nobody gave a thought to Greener.
But in those two decades the problem of climate change has forced itself upon us, and in more recent years we have come to appreciate that flying, especially over short-to-medium distances, is one of the worst contributors of all to putting huge amounts of the greenhouse gases that are causing global warming directly into the atmosphere, with rail being infinitely preferable.
So Eurostar, which began in the 1990s with one Unique Selling Point – speed – finds that eventually, when it has built its high-speed line through east London and north Kent, renovated St Pancras to replace Waterloo after 13 years' service, and cut the travel time between the two capitals from 2hr 35min to 2hr 15min, all at a whacking great cost of £6bn, it finds that it has another, even more attractive USP: environmental responsibility.
Stupid it isn't. It is making the most of its environmental potential, and is trying to turn itself and its trains even greener in a whole series of ways under a programme called the Tread Lightly initiative. It will try to reduce passenger carbon emissions by a further 25 per cent by 2012, by sourcing cleaner electricity supplies and fine-tuning the train control systems, and is looking for an array of improvements, from recycling the on-board waste and water to local sourcing of food and drink.
Mr Juniper referred to this when asked if his group could be accused of "greenwashing" Eurostar. "If Eurostar's Tread Lightly initiative, or indeed its business, was about making false environmental claims, I think that would be a legitimate criticism, but we don't believe it is," he said. "We've worked with Eurostar and looked carefully at the numbers surrounding the comparative impacts of flying and going by train, and we're convinced this is a good alternative. We think there is indeed a 90 per cent saving of CO2 – which is pretty big."
When I asked him : "Wouldn't you rather that people just didn't travel?" he replied: "No, I don't think the Friends of the Earth message is that we should sit in the dark, be cold and not travel.
"Our message is that we can do all of the things we need to do, have very high quality lives and do it in a low-carbon way. And I think that's the message that will win the day. I think the message of pain and sacrifice is not one that people want to hear, and it's not one that they need to hear, because we have all the things that we need to go low-carbon – the trouble is we're not using them."
FoE is non-committal about Eurostar's offsetting of the remaining passenger emissions, a technique of which some environmentalists disapprove. But rather than paying an offsetting company to plant trees, Eurostar is supporting three specific renewable energy and energy-saving schemes in the developing world – a windfarm in Tamil Nadu, south India, a micro-hydropower project in China, and a scheme to improve the fuel consumption of three-wheeler taxis in Indonesia.
All this just to get on a train at St Pancras, eh?
So, when the Paris-bound 11.01 service did finally pull out of Platform 10, with an army of television crews in attendance (and to my own delight, a solid bunch of genuine trainspotters at the platform's end, middle-aged geezers with cameras and notebooks), it did feel rather guilt-free, as well as privileged and enjoyable.
It was 13 years to the day since Eurostar had begun its services through the Channel Tunnel from Waterloo, carrying a total of 81,891,738 passengers on the low-speed route that went around the south of London.
Now we dipped into a tunnel almost immediately, passed under London's East End and seven minutes later came out alongside Rainham Marshes, the great flatlands of the Thames Estuary teeming with birdlife. By the time we had crossed the river and reached the North Downs we had hit our maximum speed of 300kph – 186mph. We flew past trains on parallel lines. No scheduled train service had ever hit this speed in Britain before.
Many of the people on the first train were invited guests of Eurostar and Friends of the Earth, including many people involved in environmental improvement schemes. All those I talked to, from Women's Institute activists in Gloucestershire to Hertfordshire scouts, thought the whole thing was a great idea – environmentally more than anything else.
By my watch we made the Gare du Nord at 11.18, perhaps a couple of minutes late, but who was complaining?
Even the French had something to cheer about. As you come out into the station concourse in Paris you are met by a great banner showing Napoleon urging on his troops in what was to be the battle which proved his undoing. Now the name of it will no longer be an irritant to French passengers every time they reach the London terminus. "Oubliez Waterloo!" it said, speaking of the charms of St Pancras.
You can forget Waterloo, now, chaps – and travel to London without harming the planet as well.Reuse content