At the station
Make a reservation for a bed/couchette on the night train from Paris (Gare d’Austerlitz) to Bourg-Madame on the French-Spanish border in the Pyrenees. It is cheaper to travel on a Monday afternoon, Tuesday, Wednesday, Thursday, or Friday morning; and if you are over 60 years of age your ticket will also be discounted a further 25%. For example,Tues July 11, train 473 from Paris Austerlitz, dep 21H 59, arr Bourg-Madame 7H 55; 626 FF (834FF - 25%) ($138.68) for two adults 'Decouverte Senior' 2nd class (request lower bunk in middle of train e.g. places 61, 62). First-class couchette would cost about an extra $50 per person. Arrive at the station an hour before the scheduled departure of the train (you will likely have to pass through a ticket control at the departure platform/quai). You can wheel your bike to the carriage/wagon you will be travelling in, and then compact and store your bike in the bicycle bag.