For long journeys (Paris-Toulouse in 4h30, Paris-Nice in 5h45) or journeys with a connection, the carbon footprint remains very favourable to rail. The question becomes how much time is acceptable for the company: beyond 4 hours, the decision is made case by case, and the night train is an option on some routes.
Read the source article: Train or plane: CO2 and actual times for 6 journeys from Paris