Your Pass is valid on the trains run by the national railway companies. You can hop on and off most trains as you please with your Pass, but some trains, especially high-speed and night trains, ask you to buy an additional seat reservation before you can jump aboard.
Seat reservations aren’t included in your Pass, but for a small fee, they guarantee you a comfy seat on busy routes across Europe. Trains that ask for reservations normally save you time and have lots of extra features, like Wi-Fi and power sockets or sleeping facilities, plus you can bag a space next to your friends.
You can use the link below to find information on how to check whether reservations are required, how to make them, how much they cost, and how to avoid them whenever possible: