Many years ago I went skiing and travelled from Winchester/Waterloo/W East/Dover Marine/Ostend/Aachen/Stuttgart/Ulm to Sonthofen in Bavaria. That took over 25 hours. The last part was on rock hard seats on a DB local DMU.
I suppose it is possible to travel the old Orient Express routes to Istanbul though you would need to change a few times nowadays.
Yes, indeed!
About four years ago we travelled from Oxfordshire to Istanbul (-and on through Turkey to near the Syrian border) all by train.
However we did do it as a holiday, so it wasn't all in one go; we stayed overnight in Venice, Ljublijana (Slovenia), Zargreb (Croatia), Belgrade (Serbia), & Sofia (Bulgaria).
So it was very "long" in terms of total distance, but not so "long" in terms of hours travelled in one go. We eventually flew back from Amman in Jordan.
I think you'd have to take a different route down through the Balkans now as there don't appear to be any Venice - Ljublijana cross-border trains between Italy & Slovenia anymore.
In terms of overnight travel, we had a couchette from Paris Bercy to Venice; a sleeper from Sofia to Istanbul; and another sleeper from Istanbul to Konya in central Turkey. And a final sleeper from Konya to a place called Gazientep near the Syrian border.
Of the lot, the Turkish sleeper from Istanbul was the best: modern, clean, and air-conditioned and the train had a restaurant car for dinner & breakfast too!