Comments by "Doncarlo" (@doujinflip) on "Why the Chips Get Hot" video.

@AaronOfMpls The trains fit tightly in the tunnels because that was how they also ventilated the underground. Remember they were first built when air conditioning was barely developed and they still didn’t standardize what voltage and frequency to supply electricity to all customers on. It was marginally effective but increased signage and station expansions screwed with the airflow, while increased ridership requires additional air handling than what passing trains alone could bring all the way through the tunnels.
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