Not sure about tinkering with the rail segments once you bought the ticket. But since your presence/absence on the train in your booking cannot be recorded/checked for certain (often there are no controllers, especially in CH and in D, less so in F) you can simply skip the rail segments you don't need. And possibly buy a separate rail ticket on a different day.

You mention a significant difference between the rail-less CDG and the air+rail French destination. That' s quite surprising since you could just book a return ticket to that destination without ever going there and stay in Paris. Or even train to that destination and do in instant u-turn to Paris 😃. All this because rail "check-in" is not strictly enforced.

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