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Can someone who works in rolling stock explain to me why European subway systems don’t have AC? Is there a hidden cost to converting a line to AC that I’m unfamiliar with?

Two reasons. Uses more power so may have to upgrade power supply or run less trains. In deep level tunnels ventilation limits for exhausting hot air removed from the train. The heat has to go somewhere. Historically no real need for it as it rarely gets that hot in northern europe.