Surprisingly the European 'Interrail' (various spellings even officially, one word, two words, one r, two r's) has not featured above. This has gone through various iterations, almost annually varying its requirements, since coming along suddenly in the early 1970s. Initially it was age limited at 21, for one month only, being aimed squarely at university summer vacationers, no first class, no reservations, go when you liked all across Europe by train. And very many did. Went around a very standard set of Spain to Scandinavia, staying in youth hostels (which every major rail station neighbourhood seemed to have). Most were still very traditional trains, locomotives and lengthy strings of passenger cars, always got a seat, and notable chance encounters with one another.

50th anniversary of interrail | interrail.eu

There had been a surprising number of "Student Charter" flights between various major European cities developing then, things like London to Madrid, organised by student travel groups and generally using the Mediterranean holiday airlines on midweek days when they weren't fully programmed. In a few years Interrail killed this market stone dead.

And at the other end of the day, you'd see a mix business folk in suits and the soon-to-be-drunks buying booze at the newstand in the station, the suits boarding the trains with their miniature bottles, and the others dispersing throughout the neighborhood.

My most enjoyable train ride was during my childhood.
Back then, we traveled from Singapore to Malaysia by train and I enjoyed sleeping on the bed and looking at the scenery at night.
It was sheer fun. Until today, I still have fond memories of my childhood's train trips.

The Eurostar high speed train from Paris to London and back. Drinking bubbly in the cafe car flirting with the French server at 250+ kmh........

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My most memorable is probably on the Durango-Silverton railroad in southwestern Colorado; a good amount of you might have been on it. What makes it more unique for me is that my brother-in-law and I took the train from Durango only as far as the Needleton stop, which is just a trailhead in the middle of the wilderness. From there, we hiked eight miles into Chicago Basin, where, over the next three days, we summited three of the official list of Colorado 14ers - Mt. Eolus, Windom Peak, and Sunlight Peak. Once we hiked back to Needleton, we retrieved the six-pack of beer that we had left in the creek (tied with guyline to a close-by tree) and then got to watch the other passengers shriek in horror as we boarded the train unshowered and took the ride back to Durango.

Other candidates:
Night train from Tangier to Marrakesh
Two trains from Budapest-Bucharest-Istanbul
Train from Yangon to Mandalay in Burma

I've ridden the train twice between Moscow and Beijing, along the trans-Siberian route.
The first time was in 1987. I met my brother in Hong Kong (then still a British colony), and we took the Kowloon-Guangzhou train, operated by the Kowloon-Canton Railway and the Chinese Railway to Guangzhou. Then a CAAC Trident to Guilin and a 737 to Hangzhou. From there it was by rail all the way to Moscow: express train to Shanghai and overnights Shanghai-Xian and Xian-Beijing. The Beijing train was pulled by an express steam locomotive from Xian to Taiyuan; I rode only once more behind an express steam locomotive, in South Africa in 1991. The Beijing-Moscow train was the weekly Chinese one via Mongolia, taking 6 days. This was in the Gorbachev era, when the government tried to curtail alcohol consumption. Thus there was none to be had on the diner inside Russia, so we brought along enough beer to wash down our meals. We had a two-bed cabin with a bathroom between each pair of cabins containing a hand-held shower. During the stop in Ulan Bator I walked around the station to lay claim to having been in Mongolia. At the crest of the Urals there is an obelisk with "Asia" inscribed on one side and "Europe" on the other (both in Russian/Cyrillic). From Moscow we flew on an Interflug Tupelov 134 to Berlin and a British 737 to Cologne.
The second time was in 2004, from Moscow to Beijing on the weekly Russian train via Manchuria (a 7-day trip). First I flew New York-Paris-Berlin on Air France (777/A318) and then went by rail to Moscow in a first-class sleeper on the "Moskva Express" via Minsk, where I walked around the station and can now say I've been to Belarus. As we approached Moscow the conductor asked whether he could keep my ticket, probably to be able to collect a bribe from a future passenger without a ticket. I noted I would need to show a ticket to leave the station, and the conductor offered me another ticket for that purpose. I met some friends in Moscow, And we took the "Baikal" to Irkutsk for a short stay on the Lake Baikal shore and then the "Vostok" to Beijing. This was the Russian train with a Russian diner. We had two-bed cabins with the bathroom/shower down the corridor. From Beijing we took a China Eastern A-300 to Shanghai and an Air France A-340 to Paris and 777 to New York. In Shanghai the connection from town to Pudong airport is by magnetic-levitation train (top speed 268 mph).
Unfortunately the Moscow-Beijing trains stopped running during the pandemic and are now (perhaps) being re-instated, along with the Moscow-Pyongyang through car.

In 2010 I flew from New York to Berlin on Air France (A-380/A-320) and wanted to see what Ukraine looked like. The real inducement, however, was to ride the obscure little train from Odessa to Chisinau, in Moldova, via the presumptuous, self-declared independent, Russian-speaking, sliver along the Moldovan/Ukrainian border billing itself Transnistria, which has long welcomed a semi-occupying contingent from the Russian military. I rode the Berlin-Kyiv sleeper, which ran via Warsaw and Lublin, and the Kyiv-Odessa sleeper. Since the Transnistrians control their quasi-state, the Moldovan authorities have no control over the border with Ukraine, and the Transnistrians are too inept to do anything, so one could enter de-jure Moldova with no controls at all, resulting in no evidence of my legal presence in Moldova. The train stops in Terespol, the "capital" of Transnistria, and I walked around the station a bit. Chisinau was interesting, and I then rode the Chisinau-Bucharest sleeper. At the Romanian border both the Moldovan and Romanian authorities checked passports and demanded a detailed explanation for the absence of any Moldovan stamps in mine. My explanation was credible, and they let me go. I then flew Air France to Paris and New York (A-321/777).