I gather, from a report made in Communist times, that the Brockenbahn was actually all in East Germany; but ran close by, and right to, the border. The Brocken summit was just within East Germany; on it was a military installation concerned with radar etc, manned by Soviet troops. Public rail services ran along the branch as far as Schierke: beyond and to the summit, there was military traffic only. Only people living in the immediate vicinity, were allowed to travel Drei Annen Hohne -- Schierke.I first visited the Harzquerbahn in 1973 when it was 100% steam.
Before I went I had to pre book hotel accommodation in the towns I wished to visit through an East German travel agency in London called Berolina Travel.
Normally westerners were put up in large Inter-hotels in big cities but I managed to get in what was basically a pub in Wernigerode. The main attraction today, the Brockenbahn, was then severed by the border and trains ran only as far as Schierke on the border.
They wouldn't sell me ticket to there though because I hadn't got the right papers. I did travel to Nordhausen and back keeping my camera well out of sight when we passed close to the barbed wire south of Drei Annan Hohne.
I envy your described travels -- I've rather missed out on the eastern German narrow gauge, of which I've done only the Harz system, on two separate visits; and the 750mm line Freital-Hainsberg -- Kurort Kipsdorf (had to forfeit the uppermost few kilometres of that one). I would have loved to experience the 600mm gauge around Friedland, about 150 km north of Berlin. In GDR days there remained only a couple of lines of this system, which had formerly been intricate and much more extensive; post-1945, the majority of it was dismantled, and track and stock sent to the Soviet Union. The surviving couple of lines were abandoned about 1970.I also visited the Selketalbahn which then had not yet been reconnected to the Harzbahn. It also started at Gernrode not Quedlingburg as it does today. Traffic here was largely in the hands of the six 0-4-4-0 mallets 99.5901-5906 which were more interesting than the large 2-10-2 tanks on the Harz.
I did return to the GDR in 1985 when the line had been extended to Stiege and have visited it several times since the Berlin Wall collapsed.
The extension to Stiege, by the way, was built to facilitate the running of coal trains from Nordhausen to a newly built power station at Silberhutte. Doubleheaded 2-10-2 tanks could be seen on these.
During my 1973 trip to the GDR I visited a number of other narrow gauge lines, most, but not all, of which still exist today. When I returned home my parents were pleased to see me as they thought I was going to get arrested and/or shot over there.
I never had any problems photographing trains in the GDR, except in the border areas, unlike when I was in Czechoslovakia.