In the last few days we have seen many posting from people who have bought rail pases and are trying to get reservations for them, and can't find any.
Well, welcome to the "wonderful" world of rail passes. It's a dirty little secret of railpasses, something they usually don't tell you, something you only find out AFTER you have spent your money on a rail pass, that reservations are often hard, if not impossible, or excessively expensive, to get.
Many premium trains (Thalys, TGV, Italian Eurostar) are not completely covered by a rail pass. They require payment of a sizable supplement, frequently called a "reservation", because a seat reservation comes with it. These reservations are much more expensive through RailEurope than from a ticket counter in Europe or online from the national rail companies, but it is often difficult, almost impossible, to get these "reservation" from the United States. If you wait until you are in Europe, from which these reservations are readily available from any ticket counter, you might find that they are sold out.
Night trains are a particular problem. Other than German Rail, very few national rail companies offer pass holder reservations online for night trains. Of course, Rail Europe has pass holder reservations at an inflated price.
So, look before you leap. Before you buy a rail pass, find out how you are going to be able to use it.