alt Hacker News> they even ordered new U-bahn without A/C
AC is fairly problematic for narrow underground tunnels. This is one upside of the massive 9m diameter tunnels being used for a lot of new underground lines; way easier to ventilate. But, ah, not something you can really retrofit.
It could be worse, however: https://en.wikipedia.org/wiki/London_Underground_cooling
That’s kinda incompetent excuse in my opinion. There are many other ways they can cool down the underground stations and trains. Would be expensive, but doable. However, given the track record of constructions in Berlin it would take like 100 years to do it for all Ubhan lines.