Purely suggested it because the OP mentioned taking the train Paris-Munich, and I always think the sleeper adds a touch of romance to a holiday. Much more relaxing, more space, slower, the option of a rather pleasant meal in the dining car.

Sure, flying is quicker. But my fondest memories of travel around Europe are onboard a train somewhere, watching the scenery go past and trying to guess what currency the man selling coffee accepts. Not sitting in an airport departure lounge having had my bottle of water confiscated.

It can be cheaper or more expensive than flying, depending on the whims of the market.

Having said all that, I often fly between London and NCL, even though the train is less than 3 hours. Go figure.