Slight irritation at Virgin train's new TV adverts – extolling the supposed environmental virtues of its fleet – led me to go to sniffing around its website.

After a bit of digging I found a page where the company has neatly worked out CO2 emissions for its various types of train. I can't help noticing that it has chosen to base its assumptions regarding the electric 'Pendalino' trains on the journey between Manchester and London – which is considerably flatter than that between Preston and Glasgow. But there's no doubting that electric trains do have an impressively low carbon footprint considering their 125mph top speed.

But reading on quickly illustrates why Virgin makes less play of the environmental performance of its diesel-powered trains. These so-called 'Voyagers' actually cover longer cross-country journeys than the Pendalinos – if you travel from Cornwall to Scotland you'll end up on one.

And with average CO2 emissions of 8870g/km they aren't exactly the most environmentally friendly of vehicles. Of course, Virgin divides that by its average passenger loading (119 per train) – but that still works out at only 74g/passenger-kilometer.

That means that two people travelling in an ultra-clean Ford Focus 1.6 TDCI (127 g/km – or 64g/passenger-kilometer) would actually be doing the planet a favour by not letting the train take the strain.