Take the train to Serre Chevalier!

You can go to Serre Chevalier via train from anywhere in Europe. On TGV - Europes site can you plan your trip and book online, unfortunately you can't use it for all destinations in Europe. If you can't find your departure in the list try DB Bahn's european timetable instead.

With DB bahn you can't get details about pricing but you can't book oneline. You'll have to book via a train office.

Trains to Serre Chevalier

From Paris departures a night train and four trains during the day. You can find the timetable here: the day table and the night table. From Grenoble and Marseille there is daily departures and the timetable for Marseille can be found here and the timetable form Grenoble can be found here.

If you choose by TGV (high speed train) from Lyon or Paris, an option could be to take the Paris-Milan line and step out at Olux, Italy. From there is a bus (30 minutes) to Briacon. You can book transfer here

French national train info can be found here.