Yes, you absolutely need a valid passport to travel to Paris on the train if you are starting your journey in the United Kingdom (e.g., via Eurostar). Since the UK is not part of the Schengen Area, full border controls are in place at St. Pancras International before you even board the train. In 2026, your British passport must be less than 10 years old and have at least three months' validity remaining from your date of departure from the EU. For non-UK/EU citizens, a visa or an approved ETIAS (Electronic Travel Information and Authorisation System) may also be required depending on your nationality. Once you clear the French border checks at the London station, you are effectively "in the EU" and can hop off at Gare du Nord without further checks. It is a peer-to-peer "must-know" that you cannot travel on a national ID card or a driver's license; the passport is the only document that will get you past the Eurostar gates and onto the rails to Paris.