The lack of air conditioning on many London Underground lines is primarily a geological and engineering challenge rather than a lack of will. Most of the network, particularly the "Deep Tube" lines like the Central, Northern, and Bakerloo, consists of narrow, single-bore tunnels built over a century ago with very little clearance between the train and the tunnel wall. Air conditioning units work by moving heat from the inside of a carriage to the outside; in these confined tunnels, there is nowhere for that heat to go. If every train were air-conditioned, the tunnels themselves would eventually heat up to unbearable levels, creating a "thermal sink" effect. While newer "Sub-Surface" lines (like the District, Circle, and Elizabeth Line) have larger tunnels and modern air-conditioned trains, the deep lines rely on outdated ventilation shafts. In 2026, Transport for London (TfL) is gradually introducing the "New Tube for London" on the Piccadilly line, which features innovative, low-profile cooling systems, but for many of the oldest tunnels, the clay soil has absorbed so much heat over 100 years that significant cooling remains a massive, multi-billion-pound infrastructure puzzle.