SNCF: train tickets from the end of May to the beginning of July on sale from this Thursday March 7. Trips in Europe managed by SNCF (Germany, Belgium, Spain and Switzerland) were already available for sale since February 26, for a period from May 23 to August 26.
Eurostar sales (UK, Belgium) are also already open. For TER, the opening deadline is set on a case-by-case basis depending on the region 3 to 5 months before the date of the journey.