Eurail passes are for non-European citizens or non-European residents only, whereas Interrail passes are for European citizens only.
IMPORTANT: The pass you choose is based on where you live, which may not necessarily be the same as your passport. For example, if you are a German citizen but live in South Africa, you will need to purchase a Eurail pass. If you are a South African citizen but live in Germany, then you need an Interrail pass.
When booking, you will be asked for your country of residence and passport details. You need to have the documentation that you used to book your pass with you when you travel with the pass.
2 European passports, residing in Europe: Use the passport of the country in which you live.
1 European passport + 1 non-European passport: Use the passport of the country in which you are living. For example, if you live in South Africa but hold passports for SA and Germany, you will need a Eurail pass; if you live in Germany, you need an Interrail pass.
1 European passport + 1 non-European passport but living in a different European country: You can use either passport but the country of residence is the one in which you live so you need an Interrail Pass (your official residency documentation will need to accompany you and the pass). For example, you live in Ireland but hold passports for South Africa and Germany – your country of residence is Ireland and you will need an Interrail pass with your country of residency stated as Ireland.