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.