I just did that in July. Packed for flying and would have flown. It was like driving without the convenience of leaving when I wanted and stopping when I wished. At least I didn’t have to stay awake the whole time.
It’s one of the nice things about Europe, I would rather take a train than fly, it’s so much more convenient. Even though now they are starting to make their security checks more like the airlines, especially if you take the Eurostar.