It depends on where you are traveling — which countries and which specific trains you choose to take.

Supplements are very rare — it’s unlikely you’ll pay any supplements, except on overnight trains or if you travel some of the privately owned railways in Switzerland.

Trains that require reservations are fairly common.  Many of the “name” express trains between major cities require reservations, although you can often avoid reservations by taking slightly slower local and regional trains.  Reservations usually cost about 5 euros.  However, some premium trains (EurostarItalia, Thalys, etc.) require reservations that cost up to 19 euros.

There’s a list, by country, of trains that require reservations and supplements with the approximate cost to buy them in Europe (they’re much more expensive to buy from the railpass websites) at www.railpass.com  Click on “info center” then “using your railpass” then “supplements..”