Do you need a railpass for a one-way overnight train between two countries? We do not plan to travel anywhere else on train.
No. Just buy a ticket. Railpasses are best for multiple trips of long distances.
Thanks. Is it best to buy on RailEurope?
Depends on what you mean by best. Rail Europe might be less trouble for buying a night train ticket, but it will be more expensive. Where are you going? For any night train starting and/or ending in Germany, you can get tickets easier from German Rail's website, online.
This thread is from 2008. Used to be posters brought up long past posts, now the system is doing it.