From what I have read, it looks like you can only book a private compartment on the French sleeper (couchette, really) trains if you buy tickets directly and not if you buy using a pass discount.
Or, to put it another way, if you book with an Interrail pass you have no choice but to be in a shared compartment.
This post indicates you can/might be able to book by calling SNCF, but it’s a year old and things change.
Can anyone advise if you can book a private compartment over the phone? My train dates aren’t on the system yet so I don’t think ringing SNCF to buy a hypothetical ticket will end well.
Is there is any other way to ensure a private compartment?
And finally . . . is this just something the booking engine can’t support, but SNCF can, or is it just a programming/loading oversight?
