Can anyone explain the difference/distinction between idtgv and tgv? Is it simply a matter of different train cars? It appears that the rates on idtgv are much lower.
iDTGV cars are dedicated cars on regular TGVs. They are not separate trains. You can choose "iDzap" cars if you want games, "iDzen" cars if you want quiet, and "iDnight" cars (not overnight) if you want to truly party. You can book iDTGVs up to four months in advance (three months for regular TGVs).
If you book regular TGVs as far in advance as possible, you can get discount fares comparable to those for iDTGVs.
Actually the trains serve southern France, not just the south of France.
The 'south of France" is generally understood to be the regions of Languedoc-Roussillion and Provence-Alpes-Cote de Azur. These are the regions which abutt the Mediterranean Sea.
Southern France, by definition, also includes the regions of Acquitain and Midi-Pyrenees.
My dealing with idtgv was that, in Paris, it was a little harder to get my ticket printed. Same train, a bit cheaper, but, in my case, the actual kiosk was hidden in a restaurant and hard to find. I did find it, eventually, and I got on the train.