Within Europe, the train is almost always the better choice. Not just on the famous routes — Paris-Brussels, Berlin-Prague, Madrid-Seville — but on most journeys under 1,000 km the train wins on every dimension that matters.
Time, end-to-end. Once you account for getting to the airport, security queues, boarding, and getting from the destination airport to the city centre, train journeys of up to about 6 hours are competitive with flights. Anything under 4 hours is almost always faster by train.
Cost. Trains within Europe are not cheap, but they are usually cheaper than the equivalent flight by the time you add baggage, airport transfers, and the productivity loss of not being able to work or read during the journey.
Comfort. Trains have legroom, working tables, real food, windows you can see out of, and (often) wifi that actually works. None of this needs explaining to anyone who has flown a budget European airline recently.
Environment. Trains produce something like one-tenth the carbon emissions per passenger-kilometre as flights. If that matters to you, the choice is obvious.
The trip itself. Train journeys go through landscapes. Flights go through clouds. The train from Geneva to Milan crosses the Alps. The train from Oslo to Bergen is one of the most beautiful train journeys on Earth. The train from Amsterdam to Berlin slides through the Dutch and German countryside in five quiet hours.
Our default for any European trip under 1,000 km: take the train. It is almost always the more memorable choice.