In the UK, with changes, Birmingham New Street - Aberdeen via a protracted wait in Lancaster after the high profile failure of a charter train climbing Shap ahead of us. I learnt my lesson and flew from Birmingham on future trips after that debacle.
From the UK, then Birmingham New Street to Utrecht, via the Eurostar from St Pancras to Brussels.
And wholly within Europe, Zurich - Lausanne via the Golden Pass train(s) felt like a long day.
You know for some reason the Caledonian Sleeper didn't even occur to me until I saw your post, but I have taken the Highland sleeper from Euston to Fort William. I guess it doesn't feel as grueling as long distance day travel as I can essentially drink until I'm ready to pass out for the night. My first trip, of several, on the Fort William sleeper getting drunk on whisky in the lounge car and then drunker on mead sitting on the sink in my berth remains a cherished memory.
Other than the sleeper, the longest distance I'm generally willing to do by direct train is about four hours. Anything more tries my patience, and I prefer to be able to break my journey after a couple of hours if possible.