AC units dump the heat "outside" into the air. There is no outside in deep tunnels. Nowhere for the heat to go. The tunnels have already heated up over the deca

AC units dump the heat "outside" into the air. There is no outside in deep tunnels. Nowhere for the heat to go. The tunnels have already heated up over the decades. https://en.wikipedia.org/wiki/London_Underground_cooling https://en.wikipedia.org/wiki/London_Underground_cooling It's not primarily about headroom.