I love traveling by train.
I almost went all the way from Scotland to Singapore by train, but not in one go. I went from Inverness to London, from Amsterdam to Berlin a couple of times, from Berlin to Romania many times (both via Czechia/Hungary and via Poland/Ukraine). I went from Beijing to Moscow (through Mongolia) and then on to Romania. From Beijing to Guilin a few times. I crossed Vietnam from Lao Cai in the very north to Saigon. I went from Singapore to Kuala Lumpur (before the central station in Singapore closed). And I went from Kuala Lumpur to Kota Bahru, close to the border with Thailand.
So I think all I am missing to complete Scotland-Singapore is London-Amsterdam, Guilin-Hanoi, Saigon-Bangkok (not possible all the way, but part of it) and Bangkok to Kota Bharu.
Also I went from Beijing to Kashgar, next to the border with Kyrgyzstan, and back! At that time there were no high speed trains, so the journey took 6 days round-trip.
I love the Bernina Express in Switzerland, the Circum Baikal in Siberia and the ride from Bandung to Yogyakarta in Java, which I did many times. I also took a lot of trains in Sri Lanka and really enjoyed them (although they were very slow).
I never took any trains in Cambodia, because the lines were being renovated at the time I was there.