do you have to reserve eurorail

EuroRail is a travel agency. Are you asking about train reservations ahead of time -- it depends.

Click on the Railpasses tab at the top of the page you're reading now. Here's a quote from one of the pages:

"Seat reservations, which cost from $5 to $35, can be made as early as two months in advance (three for Thalys and other international routes from France, four to six for Eurostar), or up to a few hours in advance. All trains that require reservations also limit the number of seats available to passholders. It is wise to reserve at least several days ahead (or as soon as you're ready to commit to a date and time) for night trains, weekends and holidays, high season, routes with infrequent service, any train you cannot afford to miss, or if you need several seats together. For instance, TGV fast trains in France and direct Paris-Italy day and night trains are popular routes that can sell out weeks ahead."

If you want to be guaranteed a seat on popular fast train routes and sleeping accommodations on any night train, reserve NOW.