To see what the site is about, I entered a return trip from Paris to Nice and got prices of approximately $250, $378 and $411 respectively for train, flight and car and 7.4 kg, 200 kg and 373 kg of carbon dioxide emitted per person. This definitely shows how expensive fuel is Europe and also how clean the electric TGV trains are. The train was also much faster than car at 5 1/2 hours vs almost 9 hours by car. The flight is only 1 hour 20 min but that doesn't include the time to get to the airport, through security, check-ins and all the other stuff you have to do when you fly. The only problem is that it appears to only be in French at the moment. Cool idea though.
[Source: Voyages SNCF]