Your railpass is your ticket.  It’s all you need Unless the specific train you choose requires a reservation.  Reservations cost from about 4 euros to almost 20 euros, depending on the country and the train.
 
Do not buy reservations before you get to Europe.  They cost about twice as much to buy from the railpass websites and if you change your plans you usually lose the money you spent for your reservation.  Buy your reservations about a day in advance (3 days for overnight trains) at the train station  — although reservations are often available up to an hour before train time.

1.  Only buy a reservation if the train you plan to take requires ir  — exception is Italy where you’ll want a reservation if it’s either required or recommended and the trip is longer than you’re willing to stand.  Most trains don’t require reservations, but the express trains between major cities usually do.  You can avoid reservations by taking slightly slower local and regional trains, but often the trains at the “best” times (8 to 10 AM for example) require reservations.

2.  There are a couple of ways to deal with reservations.  Since you can buy reservations for just about any train in Europe at any major train station, if you have a set itinerary you can buy all your reservations at once.  Another time saver is to buy the reservation for the train you plan to take when you leave a city before you leave the train station when you arrive — saves a trip back to the train station to buy the reservation.  If you want to take a train that requires a reservation and don’t have one, plan on arriving at the train station about 2 hours before departure time.