Warning: european trains now have dynamic pricing
I first travelled to Europe around 1980. Even into the early 2000's, the cost of a rail ticket depended only upon route, class of service, and date of travel. Since eurail passes generally provided marginal (if any) savings, it was reasonable to keep our options open and purchase tickets for individual trains just a few days before departure. Now european trains are priced like airline seats, so last-minute bookings can get spendy. Lesson: in the future, I'll either purchase either fixed-fare passes or pin down all travel reservations ahead of time.