Reservation Requirements with a France Rail Pass:
This rail pass covers trains that are operated by the national railway of France, SNCF.
Reservations are required for almost all domestic, international and night trains. Travelers with a rail pass may be required to purchase a special reservation for the portion of the journey that isn't covered by their rail pass. The Eurostar train isn't covered by any rail pass, but travelers with a France Rail Pass are eligible to receive a discount on Eurostar tickets.
Reservations are an additional cost to the rail pass and can be requested within 90 days of your travel date.