For domestic train travel within England, Scotland, and Wales, you do not need a passport. There is no legal requirement to show identification to board a standard national rail service. However, you should carry a form of ID if you are using a discounted Railcard (like the 16-25 or Senior Railcard), as conductors may occasionally ask for proof of age or identity to verify the validity of the discount. The situation changes if you are taking the Eurostar from London St Pancras to Paris, Brussels, or Amsterdam; since this is an international journey, you must go through full border control and present a valid passport. Additionally, if you are traveling on the Caledonian Sleeper or certain luxury trains, they may request a form of ID for check-in purposes. For standard commuters and tourists moving between cities like London and Edinburgh, your valid train ticket is the only document required to pass through the station gates and board the train.