Eurail at Christmas time

I am planning on being in Austria from 20-28 Dec and wanted to stop at a few places. Am considering buying a Eurail pass (Austria-Cz) but was wondering if the trains around this time will be too busy unless I reserve seats. Reserving seats from Australia (not Austria) can cost about $10 each - or is there a cheaper way?

Is there anything else I really need to know about train travel in Austria?