The trains are run by Italian State Railways. The easiest (to remember) way to their website is http://www.trenitalia.com this will actually change to another much longer address, when it loads there is an English button to click. It will let you do timetable enquiries but at present it won't show prices for Sardinia. Next years timetable won't be loaded until December but you can use this year's to get an idea of journey times.