Guide to Eurail train reservations
Your Eurail pass gives you access to most of Europe's trains, but it's good to know that some trains need to be reserved in advance. A reservation is a guaranteed seat on a specific train. You need to book your reservations in advance, which can be done 3 months in advance. The sooner you book, the more chance you have of getting a seat.
This guide explains everything you need to know about making reservations on your Eurail trip.