I wouldn't worry too much about not being able to speak Italian when in Italy, although I strongly recommend you learn at least the basics. One site that may help is the BBC Languages site (
http://www.bbc.co.uk/languages/italian/index.shtml ) although there are many others including those Lonely Planet/<generic tourist guide> phrasebooks you can get at your local bookstore.
Maybe in the small towns language could be an issue, but a lot of people understand at least some English (esp. younger people) in the heavily touristed areas.
A little patience, speaking clearly and not rushing, understanding and a phrase book goes far!
As for trains, I've found foreign trains really aren't that hard to master if you do some research before you go and know what to expect. There is an English website for TrenItalia here
http://www.trenitalia.com/en/index.html so make sure you do a lot of reading and timetable checking etc so you get a feel for the service.
Even Googling will help, eg a quick google showed this website which appears pretty comprehensive about trains in Italy.
http://www.slowtrav.com/italy/trains/intro.htm
You may find you can buy your train pass in Australia before you head over and that may work out cheaper. Check out the tourist information based sites such as
www.frommers.com,
www.lonelyplanet.com/worldguide etc etc. They have sections on train travelling etc. Trains in Italy range from very poor (slow, not very comfy) to good - although the Eurostar should be a good train.
Enjoy the trip, and have lots of Gelato while you're there. Mmmm! You're making me want to go back already!