By the middle of the 19th Century, London’s roads had become choked with traffic.
Charles Pearson, the solicitor to the City of London, promoted a solution, which was to run a railway under the streets. The
Metropolitan Railway opened on January 10, 1863 and became the world’s first underground railway. It used steam locomotives with special condensing engines designed to limit smoke in the tunnels.
London’s underground system was the first in the world, and much of the infrastructure in place today is well over 100 years old.
Ventilation shafts were the main means of cooling at the time of building, and this remains the case today. New trains with air conditioning are coming into service on sub-surface lines because these operate largely above ground and their tunnels are both larger and closer to the surface. Heat generated as a by-product of air conditioning can then escape. In deeper tube tunnels that heat is trapped. Air conditioning would cool carriages, but make platforms and tunnels even hotter. The tunnels and trains are simply too small to accommodate air conditioning units.