In 1980, my Ph.D. student John and I travelled to Prague to attend the ten-yearly Heyrovsky Symposium, a mecca for electrochemists. We took the train from Copenhagen, which went over to the then DDR on a ferry. Already on the railway platform in Copenhagen we had to show our passports. It was a night train. Early morning, about 5 am, we landed on DDR soil, and the door was opened on our compartment. Two men came in, and one of them said, in German, "Customs Control, passports please!". The other bloke had a small chest desk suspended from his neck and he seemed to be taking notes. We showed our passports and answered a few questions, where we were going and why. All was in order and the first bloke thanked us and said good bye. We were about to put away the passports again, when the other man sort of straightened up and said loudly "Passport control! Please show your passports!". We had to show them again. All was still in order, and they both left us.
As an aside, later on travelling through the DDR, we were joined by a young German couple and an old lady. The couple told us about the wonderful holidays they had had in Russia. A lady conductor came and checked our tickets. It turned out the old lady had no seat reservation, and the conductor got quite agitated, "you must have reservations!". She said she could buy one now. The old lady seemed not to know any German, and I said to the young couple, can you help with the language? Because the lady was probably Czech, and the young people probably knew Russian, which is a bit like Czech. They turned away, not wanting to get involved. I guessed that they thought this might be trouble. The old lady showed the conductor her purse, which had only a few coins in it. The conductor went out and stood at the window in the passage outside the compartment. I went out, wondering what dire things might happen to the old lady, and asked the conductor, "What will you do?". She lifted both hands and said "What can I do?" - and that was that. Interesting.
Customs control on the Czech border was quite casual. I had noticed on arriving at both the DDR and Czech border, that there were soldiers passing mirrors on long handles under the train, no doubt checking for desperate people clinging on underneath, sneaking into these countries. A week later, leaving Czechoslovakia after the symposium, this time going west into West Germany, the Czechs did a cursory check (but still passing mirrors under the train), but the West Germans studied our passports minutely and asked a lot of questions, all very formal and strict.
It must have been about 1990 or so, our family was on a train heading for Germany. The train was packed with people, both in the compartments and standing in the passage outside them. We had not been able to get seats in the same carriage, so I was with the kids and Sandy one or two carriages away. As we approached Germany, I decided to get my passport out. But it was not in its usual place. Shit! I decided Sandy must have it. So I sent Lars to her, to ask for it. He pressed through the people, and came back after some time, she didn't have it. Shit! again. Where can it be? We were getting close now. I thought and thought, when was the last time I used it? Ah, it was in Germany, visiting my old work place at J{\"u}lich, where I had to show it. Ah, then I put it in a different place, my little shoulder bag, and there it was. Phew!
Still in the time when we sometimes had to show passports (in fact, recently we have to again, on the Danish/German border, especially going into Denmark, despite the Schengen agreement), I was returning from Germany, and we had to transfer to a bus, because of work on the railway lines. Before the trip I had ordered a new passport because it was now out of date. But because of a foul-up, I didn't get it before the trip. I thought, they will probably not even check passports. I don't remember why I didn't have to show it on the way to Germany - I guess it was the Danes who were more concerned at the time. So I had an out of date passport on me, hoping that they wouldn't notice that. OK, we were sitting in the bus, and the Danish policeman came through. We all had the passports out holding them up, and the friendly cop looked at us and said, "ah, you all have your passports", and left again without looking at any. Phew again!