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It's going back a few years now (2010), but I took my bike in a bag on the TGV from Lille to Valence TGV to do the Marmotte and I found it hassle-free and sparsely populated. I was hoping to do a fair bit of French rail travel next year including going from Paris to Lyon and back, so I guess I'll find out more. Can't fault the time of just over two hours and cheaper than a single from London to Brighton, though.
Have to say my experience of French trains was entirely positive when I last went and that included an overnight sleeper from Paris to Latour de Carol. There are still a fair few Intercites de Nuits and they are apparently bringing more back.
I'm going back next year and taking some more popular routes with a bike, so I may well revise that opinion. :D