During the late nineteenth and early twentieth centuries, networks of pneumatic tubes propelled mail under city streets. Philadelphia, Pennsylvania, introduced the first pneumatic tubes in 1893. Boston, Brooklyn, New York, Chicago, and St. Louis soon followed with similar systems. By 1915, these six cities had more than fifty-six miles of pneumatic tubes pulsing invisibly beneath their streets, connecting main and sub-post offices.
The network relied upon compressed air. Cartridges containing mail were loaded into airtight tubes, and compressed air propelled the cartridge to the end of the tube. Cartridges could also travel in reverse by means of suction that created partial vacuums in the tubes. The underground pneumatic tube network was very popular in large, congested cities, but tube installation could be problematic and expensive. Tubes competed for space with sewage or gas pipes, and they had to avoid high underground water levels. Private companies built the tube systems, and they then rented them to the Post Office Department.
Nancy A. Pope, National Postal Museum