Seat61 is a good website, but it is a a mix of a fansite that got commercial. In that sense, its author proposes journeys that are absolutely ridiculous as a practical matter, like travelling from London to Rome by train.

He also downplays the hell that is station transfer in Paris via subway, particularly if you have luggage, and usually understate the danger travelers incur if they buy tight connections with different train companies - one gets late, you lose your connection.

That site has a great selection of links and how-to-do guides for buying tickets online, but I'd not jump in the wagon (pun intended) of its "fan" side that promotes 16h train journeys as opposed to 2h30 flights as normal decisions. He also ignores the fact more and more train companies are curtailing seat pitches in 2nd class.