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.

The lack of air conditioning on much of the London Underground, particularly the "deep-level" lines like the Central, Northern, and Piccadilly, is a result of Victorian engineering challenges. These tunnels were built over a century ago and are only slightly larger than the trains themselves, leaving no physical space to mount bulky AC units on the exterior of the carriages. Furthermore, AC works by moving heat from inside the carriage to the outside; in a cramped, deep-level tunnel with poor ventilation, this would cause the tunnel temperature to skyrocket, potentially reaching dangerous levels for the entire network. While the newer, "sub-surface" lines like the District, Circle, and Metropolitan have full air conditioning because their tunnels are larger and allow heat to escape, the deep lines rely on outdated fans and "forced air" systems. To fix this, Transport for London (TfL) is slowly introducing new "Inspiro" trains with innovative, compact cooling systems, but the sheer cost of retrofitting the narrow, ancient tunnels remains a massive logistical and financial hurdle for the city's infrastructure.