At Alexanderplatz, a wall was demolished and a whole hidden section of the underground station was revealed, which surprised many local people, who had no idea it was there. Many Berliners took special trips just for the pleasure of getting on and off at stations which had been forbidden to them for so long. Fairly soon as well, any station which had a name that was 'too communist', had it changed. Finally, after reconstruction which lasted several years, the disused section of Line 2 came back into use. The two markets at Nollendorfplatz and B lowstrasse were banished elsewhere, and Wittenberplatz was again connected to Th lmannplatz, now renamed Mohrenstrasse.
Mohrenstrasse station has another secret, by the way.
If you go there, you see its walls and pillars are decorated with a very lavish red marble. The answer is they didn't. Go to the Fifth essay in this series.