Originally Posted by haveric
I agree unless you're taking one of the night trains between these cities. Many of the trains only have 1 or 2 cars of couchettes, and can be booked far in advance, particularly in summer. DO NOT book, however, until arriving in the Czech Republic (or at least Europe) -- prices are much cheaper than in the USA.
I did a similar trip in '03, travelling from Prague - Warsaw - Krakow - Budapest - Venice - Vienna and found the train a great way to go.
I would second this advice, as I just took trains around central Europe, including from Prague to Vienna to Budapest. My Prague to Vienna train was 841 Kc, which works out to around $35 or so. (I can't get to Philly for that price.) Just arrive early to stations, because it sucks to have your train leave without you while you wait in line. You can use the time to consider writing down where you want to go and what train you want to catch on a slip of paper if there are likely to be any language difficulties.
The advice about booking sleepers in advance is also wise, although you probably would only need one if you went from Prague to Budapest or Budapest to Ljubljana or Trieste Or Zagreb.
Two more things:
Consider adding Vienna to your plans, if only because it's so close to the other cities you mentioned (and, as most people are surprised to learn, is east of Prague).
Being of Hungarian descent, I have to point out that Hungarians are not Slavs, but Magyars.
Sounds like an awesome trip.