It really depends on needs .... the amount of travelling,, where that will be, and how much flexibility is needed. Two stand out countries are UK and Switzerland - some form of rail pass covering those countries can easily pay for itself. Especially if you want some flexibility to deal with variable weather. But anything last minute in (Western) Europe can be expensive. But a well planned journey with individual sectors pre-booked can be a lot cheaper. Eurail pass = travel insurance
Take the UK this coming Monday, most trains for the 4.5 hr journey from London Glasgow are well over $200 AUD. To Cardiff (2hrs) for $150AUD. The one hour/100km journey to Brighton, $60 AUD. Imagine paying $60 to get from Melbourne to Geelong or Sydney to Wollongong!
Now at the other end of the spectrum, Luxembourg was mentioned up thread. Within the confines of Luxembourg (yes there are places you can go via rail within the small Grand Duchy), a railpass is 100% useless - as public transport is free.