See National Rail to plan routes and check fares.
I'd start by checking round-trip fares from Australia to Europe, as those will be the most expensive ones.
You can also check timetables and fares on the website of Spanish rail network Renfe (www.renfe.com).
Seabus attendants may check fares, but do not conduct enforcement.
Coincidentally I was checking fares this morning for a short trip to Florence in early November.
Ask American whether award seats are available when you wish to travel (there are often blackout periods in summer months), then check fares.
Hang on to your ticket until you get off, as inspectors almost always board to check fares.
Another good website to check fares from Australia is www.travel.com.au.
To check fares, you'll need to find the train operator website for each country, there's a list on the Seat 61 website.
Check lines and fares on www.renfe.com.