I got to know that while east Berlin has a relatively dense tram railroad network in the city, West Berlin does not because it was mostly dismantled during the cold war. Both parts inherited the tram network built before the partition of Germany.
Why did West Berlin so? Because busses are more flexible for a little political enclave?
After the end of WWII the technology was put back in order and started carrying passengers relatively quickly again, leading to something resembling a network at the start of 1946. But of course it was still a precarious situation, spare parts were rare, power outages were happening from time to time, … Then came the blockade in 1948 and the tram system in the western half of the city was hit hard, many lines were cancelled for the time being, the rest had greatly increased intervals between trains and no trains traveled after 18:00h. From March of 1949 on the personnel of the trains changed at the sector border to the Soviet sector. In August the BVG (the public company running the trains) was split in two between the western and eastern half of the city, leading to more and more lines not crossing the border anymore but staying inside their zones (no trains were crossing after January 1953).
In 1954 the western BVG started to slowly replace the tram system by enlarging the already existing underground system, which was deemed more modern, and by establishing more bus lines, which didn't need as much space (street traffic at the time was increasing rapidly) and maintenance as the train tracks. There was never a formal decision to do so, but several smaller decisions led to it. The leading cause was possibly the new prototype train TED 52 which was deemed unsuitable (it was a one way train, which couldn't be used on many of the new lines) and consequently not purchased (the already existing budget was instead spent on more buses). This led to the tram trains becoming older thus less reliable and ultimately obsolete. More and more lines were replaced and in 1967 the last line was closed and it would take nearly 30 years and the reunion of the separated city before there were again trams running in the western half.
The tram system in the eastern part of the city has also been reduced in the 60s and 70s but the planned expansion of the underground system couldn't be continued because of economic reasons, which led to the trams remaining an important part of public transport.