Below London's streets. Illustration of the network of pipes and tunnels beneath London, England, in around 1880. The railway system (lower centre) is the Metropolitan railway, which preceded the London Underground system. The lower horizontal pipe is carrying the Post Office's pneumatic package transporter, a small train that ran on compressed air. It was used to carry cumbersome postal items around London. The pipe above it is a water pipe. The pipes shown in cross-section just below street level are carrying gas. The larger pipes to either side of the railway tunnel are carrying sewage.