New York Post Office - Pneumatic Tube
Photograph shows the Pennsylvania Terminal Post Office (General Post Office Building), now called the James A. Farley Building, located at 421 Eighth Avenue, New York City. (Photo: Bain News Service)

United States Post Office Pneumatic Mail Tube Terminals 
General Post Office New York NY

5,000,000 letters sent daily throught underground Pneumatic Mail Tubes in New York city.
Sequi-Centennial, Philadelphia 1926.

The Pneumatic Tube Service New York - Brooklyn 
At the picture has been mentioned: Photo Courtesy Albert Goldman Postmaster, New York City

Sources and More Information

• The Pneumatic Mail Tubes: New York's Hidden Highway And Its Development.