I've done one rail day trip - to Köln.
Ryanair from Manchester to Charleroi (£14.99 each way), local train to Liege via Namur, and on to Aachen on an ancient SNCB EMU.
Then a DB RE to Köln, and after a bowl of goulash soup at the Hbf an ICE back to Brussels Zuid on the Belgian HSL.
Finally a local from Brussels to Charleroi (via the real Waterloo!) and Ryanair back to Manchester.
It rained all day, and it was foggy (it was November).
But it ticked off a gap in my rail knowledge between Brussels and Köln, and I was impressed by the industry along the Meuse/Maas and the brief views of the Ardennes.
I've given up crack-of-dawn flights.
Instead I like 3-4-day trips, fly out to A, train to B, C, D on different days and fly back from E, usually involving several countries en route.
I've just got back from Hamburg-Kyiv via Berlin, Warsaw, Krakow and Lviv. Highly recommended for the variety, interests and contrasts between DE/PL/UA.
Can recommend European fly-rail whistle-stop trips with opportunistic sight-seeing during the overnight stops.
You also learn the best/cheapest way to travel in each country, and in my case any discounts available to seniors.