One further point...
If you are using a pass, be aware that it will not guarantee you a seat. Often this isn't a problem as one car or more on cross country trains are generally designated at non-reserved cars. However, if there are railworks, floods, big cricket/football/rugby games or a big concert, the trains can be packed.
Thus, if travel plans coincide with a big sports event or concert, or holiday (different depending on the country/city), you should try and get a seat reservation if you need to take a particular train. Otherwise you could be standing in the aisle or between cars.
Also, the luggage spaces tend to fill up quickly, so if you are traveling with large or multiple pieces of luggage, you might want to avoid peak travel times.