This reminds me of a rail trip I’ve always wanted to take: Western Europe to Singapore, which may not be as geopolitically interesting, but may be the longest possible continuous rail journey.
Unfortunately you may have to wait some time, at the moment the journey is not be completable because the Paris-Moscow express service (and indeed all train service between Russia and Western Europe) is suspended due to sanctions against Russia.
When I first had the idea there was still a gap in the way in Southeast Asia, but it looks like it may have been closed now: https://www.seat61.com/map-of-train-routes-in-southeast-asia...