What does the night train rule mean?

If you have an Eurail pass and are traveling on a night train without continuing your journey from the arrival station, only the day of departure is counted as a travel day.

For example: If you board the Nightjet from Amsterdam to Vienna on April 26 and arrive on April 27, it will count as 1 travel day, which is April 26. However, if you continue your journey on April 27 by taking another train, you must use a 2nd travel day. So, if you travel from Vienna to Budapest on April 27, it will count as a second travel day.

Before boarding the night train on the first travel day, you can use your Eurail pass unlimitedly. This is because it's the same day as the one you use for your night train journey.