The most eco-friendly means of transport? The train, of course

Bologna–Naples takes 3 hours by train; by air it takes at least 15 minutes longer. Emissions are 88 kg of CO2 lower (25 kg against 113 kg).
On Rome–Venice, too, the train wins: -78.3 kg of CO₂ and -29.2 litres of petrol (the lower consumption is not to be underestimated either).

Now let’s look at connections outside our country.
From Milan to Zurich you gain 15 minutes compared with a journey by air and, at the same time, you also use 102.2 kg less CO₂ and 39.8 litres less petrol.

On the Turin–Paris route, the train is slightly slower: 5 hours and 40 minutes against 4 hours and 44 minutes by air. The CO2 saving is nevertheless considerable: the train’s emissions are 37 kg, less than a quarter of those of the aeroplane (166 kg).

If you are in Paris and want to reach London, the train is without doubt the fastest means. Going through the Channel Tunnel, you arrive in 2 hours and 16 minutes, a full 3 hours and 24 minutes sooner than by air. Emissions are also 32 kg of CO2 lower (26 kg against 58 kg).
Among the journeys examined in a recent comparative study of trains and aeroplanes, there are numerous routes on which the train is convenient and environmentally friendly: Brussels–Frankfurt, Paris–Geneva, Amsterdam–Paris, Amsterdam–Düsseldorf, Madrid–Valencia.

Do not forget the comfort factor either. On a train the living space per passenger on board is greater; you can get up and walk around much more easily. In general, the comfort is certainly greater. Not to mention the possibility of staying connected to the Wi-Fi network throughout the journey, to work, watch films and TV series, consult travel guides, make phone calls (the latter without disturbing other passengers, if possible).
So, all aboard, and enjoy your train journey!