http://autotrain.voyages-sncf.com/dynamic/autotrain-webapp/homeControl.action. here is a link to the french booking site that i always use. just fill the right hand side,
Aller simple one way
Aller-retour return
Votre gare de départ where you want to go from
Votre gare d'arrivée where you want to arrive at
Aller le * outward date
Retour le return date
MOTO / SCOOTER
Je certifie que ma moto ou mon scooter respecte ces dimensions. tick this box to certify your bike is no taller than 1.55 m.
then click Rechercher to search prices.
this will give you the prices for the service. you can then book from there, and they send the tickets to you either by post or email all very easy.
Some more tips. I always leave my panniers and top box with all gear in and just remove tank bag to take on passenger train, in the four or five times i have used the service never had any damage or stuff go missing, you can get extra insurance when you arrive at bercy to load the bike, just go into the office, as i recall last time it was 20 euro for 3000 euro cover.
if you go to Avignon via bercy there is a cheap F1 hotel in walking distance from TGV avignon, we stay there the night and get them to book us a taxi for the morning to go and collect the bike from the autotrain station thats about 7k from the hotel, taxi was about 10 euro.
It all may seem a little daunting but do it and you will find its easy. Bercy is a 10 minute walk from Gar De Lyon.
they also do a train to Biaritz that will get you very close to the pyraneese and spain, we will probably use this on next years trip.