If you're travelling with an Interrail Global Pass, you can use it to make 2 journeys in your country of residence. 1 of these journeys can be used for outbound travel at the start of your Interrail trip. The other journey can be used for inbound travel at the end of your trip.
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Outbound journey - to be used on your*first*day of travel:
Travel from any location in your country of residence to its border, or to an airport or port within your country. You can travel with more than one train, provided the outbound journey is completed within the same day.
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Inbound journey - to be used*on your*last day of travel:
Travel to any location in your country of residence from its border, or from an airport or port within your country. You can travel with more than one train, provided the inbound journey is completed within the same day.
The journeys must be made during the overall validity period of your pass. If you have a pass with flexible travel days, such as 5 days within 15 days, and you use your outbound and inbound journey, you'll have 3 days left to travel outside your country.*Check out this example trip from Berlin to Amsterdam.
Please note that your outbound and inbound journey cannot be used to cross your*country halfway through your Interrail trip.