Conventional air conditioning was initially ruled out on the deep lines because of the lack of space for equipment on trains and the problems of dispersing the waste heat these would generate. Different systems were proposed to cool Underground trains, including the use of large blocks of ice inside the train.

Most of the London Underground (the "Tube") lacks air conditioning due to the profound engineering challenges of the Deep Tube lines. When these tunnels were bored in the late 19th and early 20th centuries, they were designed to be as small as possible to save costs, leaving virtually no "headroom" between the top of the train and the tunnel ceiling. Modern air conditioning units require significant space on top of or beneath the carriage, which the deep-level tunnels simply cannot accommodate. Furthermore, air conditioning works by moving heat, not destroying it; an AC unit on a train would pump the hot air out into the tunnels. Because the London clay surrounding the tunnels has become saturated with heat over a century of operation, it no longer acts as a "heat sink," meaning the tunnels would become dangerously hot for passengers waiting on platforms. While the newer, larger "S-Stock" trains on "cut-and-cover" lines (like the District and Metropolitan) do have AC, the deep lines (like the Central and Northern) rely on improved ventilation shafts and ground-water cooling systems until "New Generation" trains with compact cooling are fully deployed.