When does it make sense to combine a rail pass with train tickets?
There are several situations.
Like when the bulk of your travel is taking place within a certain geographical area (1 country, 2 countries or more), but you have one rogue train ride needed for an additional country. Then, combining a rail pass and train tickets may be the most economical option.
Another example is when you’re taking an international high speed train such as the Thalys or the Eurostar. These trains require travelers with a rail pass to purchase a reservation, but their passholder fares may be priced fairly high. In fact, booking regular train tickets in advance may be more cost effective than using your pass benefit. Also, for many of these high speed trains, purchasing the passholder discounted fare will cost a rail travel day. Sometimes purchasing a separate discount advance purchase ticket is your better choice all around.
Yet another situation would be when you want to take an international trip that starts and arrives in a country covered by your pass, but travels through a country not covered. An example would be traveling from Krakow, Poland to Budapest, Hungary. Those trains will actually travel through the Czech Republic and Slovakia as well. If your pass doesn’t cover all of these countries (Poland, Czech Republic, Slovakia, and Hungary), you are not eligible to purchase a reservation on the missing ones. You will, however, be able to buy the stand-alone tickets you need to bridge your journey.
Lastly, if you are traveling on a route that is operated by a private rail company, this may not be covered by your rail pass and will require the purchase of a separate train ticket.