Border crossings by night train are generally easy if you are well-prepared and have all the necessary documents.
Carry a valid identity card or passport. Please note: Some countries, such as the UK, require a passport, especially if you travel from a Schengen country to a non-Schengen country. Also, check whether you need a visa for your destination country. This depends on your nationality and the destination country.
Keep your travel documents and tickets handy and safe at all times. Passport checks usually take place on board the night train, either during the journey or at a station near the border. In some cases, there may also be customs checks.
Especially if you travel to unfamiliar countries, you should find out in advance about the current entry regulations and any special requirements when crossing the border. The websites of the respective railway companies or official government websites are helpful here.
The author once had the unpleasant experience of the train attendant demanding the then-physical Interrail ticket and passport overnight and keeping them with him. The next morning, the Interrail ticket could no longer be found. Personally, the author would never hand over his ticket for a longer time.
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