I have train tickets from Rail Europe and most (e.g. Prague to Vienna, Vienna to Budapest) are open tickets. I was searching the forums and some say that you have to exchange the tickets for "real" tickets. Is this true for all tickets? How can you tell if you have to exchange them? Rail Europe's FAQ says if you got paper tickets mailed then you don't need to do anything so I'm a little confused. I can take a picture of my tickets if that's helpful.
Can I just hop on any train on the route I'm taking? As I understand, there are different train operators.
(Please don't tell me how I should not to buy tickets from Rail Europe; I already bought them and they were bought with hotel and air together. The prices were comparable and I didn't know where my tickets were coming from.)