Whether you need a passport for a train to Paris depends entirely on your starting point in 2026. If you are traveling from London via the Eurostar, the answer is a definitive yes. Because the UK is not part of the Schengen Area, you must pass through both French and UK border controls at the station (St. Pancras or Gare du Nord), where your passport will be checked and stamped. However, if you are traveling to Paris from within the Schengen Zone (e.g., from Amsterdam, Berlin, or Brussels via Thalys/Eurostar), you generally do not face a formal passport check due to the principle of free movement. That said, in 2026, many European countries have reintroduced "temporary" spot checks on trains to manage migration and security, so it is a legal requirement to carry a valid national ID or passport even when crossing internal borders.