My first visit to Czechoslovakia, in 1991, was interesting. The train to Liberec (Cz) left from its own platform at Zittau (De) and to access it you had to pass through the border control, a hut a short way along the platform. I was a bit nervous, on my own and not speaking a word of Czech. The severe looking lady in the hut did not help this: after spending a minute or two puzzling over my UK passport, she then got up and took it into another room. However as soon as she got up I could see that on the wall immediately behind her was a large pin-up of a naked girl with big boobs. Said lady (the clothed one) then returned with a big smile, handed back my passport and said, "Welcome to Czechoslovakia!"
The train was also unexpected for an international (if really only local): 5 x 4-wheel railbuses joined together (probably motor+trailer+motor+trailer+motor), of the type now known as class 810 and once ubiquitous across former Czechoslovakia. As we bounced along the jointed track from Germany into Czechoslovakia, and brieflly in and out of a bit of Poland, I could see that the windows of our full carriage were plastered with children's drawings. A lovely and unusual introduction to a strange country.
Some time later another interesting border crossing was one of the two that sprang up around the turn of this century on either side of Sighetu Marmatiei in the far north of Romania, to encourage travel to newly independent Ukraine. The one to Rakhiv to the east had already closed after a very brief life, but the line to Teresva to the west was still clinging on with two train pairs on (I think) 2 days a week. The train was a UZ dmu (the tracks were dual gauge within Romania for historical reasons) and it was reasonably full. My ticket was, unbelievably, an Edmondson, the last one I ever received on a "normal" railway, and probably the only one still issued at Sighet which had gone "computer" by then.
At the Romanian border post our passports were collected, and returned after the usual wait. However mine was personally delivered by a stunningly gorgeous (young, female) official who informed me, gravely, in immaculate English, that she would allow me to proceed but could not guarantee that I would be allowed entry into Ukraine. I said I would take the risk (resisting the temptation to get off and propose marriage) and off we set, across the river Tisa bridge which marked the border with Ukraine. However, immediately at the far side we stopped again and everyone alighted: this was the Ukrainian border control, which I was not expecting here so was last off and into the queue in the hut down the embankment. Passport check was rapid until it got to me; everyone else was a local with empty bags after selling their wares in Romania. However at this point a scruffy bloke entered the hut and shouted something; luckily I realised he was the train driver checking that no one wanted his train! I shouted "tak!", got my passport stamped quickly and hurried back onto the train. I was the only passenger for the rest of the trip to Teresva, all the rest having made their way on foot from the border point.
Shortly afterwards this service was withdrawn too, after reports of huge smuggling operations and damage to trains (as well as to the local economy, according to the mayor of Sighet). This was once a busy international crossing, in the days before WW2 when Teresva was part of Czechoslovakia. Last time I visited it was closed and derelict, though possibly since revived for transit since the start of the Russia-Ukrainian war? The line to Rakhiv certainly had a brief and well-publicised rebuilding, and led to a (very) brief, token passenger operation a year or two ago.