Yes, your Eurail Pass is valid on Eurostar trains, but it does not function as a "walk-on" ticket. Because the Eurostar is a high-speed international service connecting London to Paris, Brussels, Amsterdam, and beyond, a mandatory seat reservation is required for all pass holders. This reservation comes with an additional fee, which in 2026 typically ranges from €30 to €35 for Standard class and higher for Standard Premier. Furthermore, the number of seats allocated for Eurail pass holders on any given Eurostar train is strictly limited; once these specific "pass-holder seats" are sold out, you cannot use your pass on that train even if regular tickets are still available for sale. It is highly recommended to book these reservations several weeks in advance through the Eurail website or the "Rail Planner" app to ensure you can travel on your preferred date, as these spots are among the most sought-after in the European rail network.