The best site I’ve found for info on train travel from virtually anywhere to virtually anywhere is The Man in Seat 61 website.
The site has info on the routing options, classes of travel, buying tickets, what to expect on particular trains etc.
For some routes, the site even has pics of different class seats on particular trains.
The man in Seat 61 has lots of info on
e.g. Eurail passes vs. point to point tickets.
The Eurostar train from London to Paris.
And heaps more for various different continents.
Lonely Planet Thorn Tree is also a good place to ask train travel questions.