Take a plane, as there are not many international connections over long distances. Low cost airlines are cheaper than train and they are fast. People often don't realize the distances and speed. While travel in the Western Europe is generally fast by train, further east you go, the distances get larger, and speed train far worse. So it will take 18 hours train ride to Split from Prague, but 1:45 minutes by plane. You will spent most of the time sitting in train and waste a time for any sightseeing.
These are really long distances and it will take ages to travel these routes by train. For instance, Prague to Barcelona will take a full 24 hours with 3 or 4 changes. Athens to Split will be even worse as there are no international trains running in Greece, and no direct train running from near the border. For example, you would need to get a bus from Athens to Thessaloniki, from there the train to Split will take over 40 hours with a 13 hour layover in Belgrade and a few hours in Zagreb. Have a look at the German rail website for all European timetables. So, unless you have the time and like long distance train travel, it's better to fly.
http://www.bahn.com
If I were you I would look around cheap airlines as trains in some countries are rather expensive. Spain - England you could look at ferries from Bilbao and Santander
I prefer trains
