Anyone who is really interested in train travel anywhere in the world, check out http://www.google.com/search?q=the+man+in+seat+61&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-GB:official&client=firefox-a, the man in seat 61.
There is a great booking agency at 1 Regent Street, London SW1, Rail Europe. I went there the other day with my boss - we're taking the train from London St Pancras down to Siena, Italy in September. Excellent service, will even sell you a carnet for the Paris metro at cost. We are going on a train from London to Paris, change heading for Florence with couchettes, and the train on to Siena the next morning, round trip £234. Yes, I know I could fly cheaper in theory, but first the fun of the travel and the time for a meal in Paris and Florence and the ability to move about and have a meal on the train as well (plus my boss will be quite pregnant at the time and can't fly anyway).