Booking directly through Trenitalia's website is easy, and a bargain compared to rail travel in Britain. There are lots of discounts available too, we were able to find Eurostar tickets between Rome and Venice for Eu29. However, the standard first class fare is only Eu74. That is the most you could possibly pay, for a journey that is the equivalent of London to Edinburgh (but is over faster).
For our convenience their website is even in English.
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