The party train used to run from Gare du Nord and was aimed at the UK market, with infalted prices to match. There is an overnight train to the Alps every (?) day in winter from Gare d'Austerlitz. 6 couchettes to a cabin. Not sure if they are direct to Briancon - might need to change in Chambery. I've certainly done midweek and weekend departures to/from Bourg St Maurice. Also worth looking at daytime TGV from Gare de Lyon, and I vaguely remember an overnight Thalys that stopped at Lille then went to the Alps somewhere.