Ergle said:
If you're travelling with your bike by train in France, you won't need a bag if you book your bike on the train. You'll pay an extra 10 Euros for this, but will be able to hang your bike on a rack provided for the purpose. I believe most trains have this facility - google Seat61 for more info.
Unfortunately, while some of the new and the refurbished TGVs have cycle carriage facilities a lot of them still don't - especially on the routes to Marseille and beyond.
You can check whether the train you are proposing to catch has cycle carriage facilities. Here are some screenshots I made earlier (perhaps for a post on this forum).
www.tgv-europe.com is probably the best site if you want to book and buy online.
If you search for trains you get a series of listings like this:
click on the '+details' tab on the top right-hand side. The expanded listing looks like this:
Unfortunately this train doesn't carry bikes (no little white bike on blue background pictogram). Here's one that does:
or at least the second one does.
If you want to plan a route using trains with bike carriage facilities then your best bet is the DeutscheBahn website (yes really - I wish I was joking). But unfortunately you wont be able to buy tickets from them.
The cost of booking bikes onto Eurostar and French trains means that a bag like the Ground Effect Tardis will quickly pay for itself.