It varies according to the routing.
Eurail passes are either accepted fully on some scenic trains in Europe or they provide a discount on a ticket.
For example: If your Eurail pass covers Switzerland and you wish to travel on the Glacier Express, your pass can be used ‘as is’ on the section between St Moritz and Disentis but, on the section between Disentis and Zermatt, it provides a 25% discount on the normal price of tickets for that section.
